I just deleted a git branch.
For example :
mkdir test
cd test
git init
echo "Viva yo" > test.txt
git add .
git commit -m "test"
git checkout -b rama
echo "Viva tu" > test2.txt
git add .
git commit -m "test2"
git checkout master
git branch -D rama
How can I recover the branch that I just deleted?
If I knew the hash of the commit I could retrieve it with:
git checkout -b rama EL_HASH
But I don't know what hash it has, I didn't look at it.
The following code seems to do the same thing with and without ampersand( & ):
<?php
class Cartera {
public $dinero = 93;
public function &muestraLaPasta() { // Si quito este ampersand
return $this->dinero;
}
}
$carteraDeMaria = new Cartera;
$miPastaGansa = &$carteraDeMaria->muestraLaPasta(); // y este
echo $miPastaGansa; // el resultado es el mismo
?>
Whether I leave them or remove them the result is the same, it prints 93. What are they for?
Thanks to Ivan Botero's answer, I know that this function returns a reference.
But it seems that it is not a reference capable of surviving on its own. If I assign it to a variable, that variable contains the value, not the reference, despite the fact that what I have assigned to it is the result of that function, which is supposed to be a function that returns a reference.
<?php
class Cartera {
public $dinero = 93;
public function &muestraLaPasta() { // Esta función devuelve una referencia
return $this->dinero;
}
}
$carteraDeMaria = new Cartera;
$miPastaGansa = $carteraDeMaria->muestraLaPasta(); // Parecería que almaceno esa
//referencia en una variable
echo $miPastaGansa . "\n"; // el resultado es el mismo
// Cuanto dinero tiene Maria
echo $carteraDeMaria->dinero . "\n";
// Cuanto dinero tiene Maria ahora
$miPastaGansa = 5000; // Pero las modificaciones a través de esa referencia
// almacenada en esta variable
echo $carteraDeMaria->dinero . "\n"; // No se propagan
I think there is still some fine detail of that reference returned by the function that is escaping me.
How come I have to use another & before assigning it? It would seem that then I have a reference to the reference to money. Instead of just a reference to money, which is what I want.
How do I put multiple occurrences of elements that are part of a sequence that is inside a complex type?
I have the following schema:
<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified" attributeFormDefault="unqualified">
<xs:element name="Measurements">
<xs:annotation>
<xs:documentation>Schema for Measurements data transmission</xs:documentation>
</xs:annotation>
<xs:complexType>
<xs:sequence>
<xs:element name="m_doc" type="xs:string"/>
<xs:element name="m_parameters">
<xs:complexType>
<xs:sequence>
<xs:element name="m_name" type="xs:string"/>
<xs:element name="m_isEngValue" type="xs:boolean"/>
<xs:element name="m_unit" type="xs:string"/>
<xs:element name="m_radix" type="xs:string"/>
<xs:element name="m_value" type="xs:string"/>
</xs:sequence>
</xs:complexType>
</xs:element>
</xs:sequence>
</xs:complexType>
</xs:element>
</xs:schema>
And I try with the following XML:
<?xml version="1.0" encoding="UTF-8"?>
<Measurements xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" >
<m_doc>TestDoc</m_doc>
<m_parameters>
<!-- Primer grupo -->
<m_name>Length</m_name>
<m_isEngValue>false</m_isEngValue>
<m_unit></m_unit>
<m_radix>HEXADECIMAL</m_radix>
<m_value>0</m_value>
<!-- Segundo grupo -->
<m_name>Height</m_name>
<m_isEngValue>false</m_isEngValue>
<m_unit></m_unit>
<m_radix>DECIMAL</m_radix>
<m_value>0</m_value>
</m_parameters>
</Measurements>
Getting the error:
Cvc-complex-type.2.4.d: Invalid Content Was Found Starting With Element 'm_name'. No Child Element Is Expected At This Point., Line '13', Column '15'.
This error corresponds to the second group of 5 elements that constitute a parameter. If I remove that second group then I have no error.
I'm testing with an online validator: www.freeformatter.com
The schema is a requirement over which I have no control and am bound to abide by. I have to make XML that conforms to that schema.
When I create a new project in git it contains one and only one branch, the master branch:
~> cd test
~/test> echo "Prueba de git" > Readme.md
~/test> git init
Initialized empty Git repository in /home/jose/test/.git/
~/test> git add .
~/test> git commit -m "Prueba git"
Created initial commit 0babea8: Prueba git
1 files changed, 1 insertions(+), 0 deletions(-)
create mode 100644 Readme.md
~/test> git branch
* master
If I clone a project I find that it may have many remote branches:
$ git clone https://github.com/git/git
$ cd git
$ git branch -r
origin/HEAD -> origin/master
origin/maint
origin/master
origin/next
origin/pu
origin/todo
And that as a result of cloning there is only one local branch, the master branch:
$ git branch
* master
Is there something special about the master branch, is it different from the branches I create with git branch? Is it mandatory? Will something bad happen if I delete it?
If the command cdin this script fails it can be a disaster:
cd $HOME/el-directorio
rm -rf algo
You could use an if to check but when a script has dozens of commands it's expensive and error prone to have to put in a check every time.
How to make bash script stop on any error?
I frequently run this command:
eval $(resize)
In order not to have to write it every time I have added this line to $HOME/.alias
alias r="eval $(resize)"
But when I run rit sets me to 80 columns, not what it corresponds to. If the alias is ready, this appears:
~> alias | grep "alias r="
alias r='eval COLUMNS=80;
That is, $(resize)it is evaluated when the .alias file is loaded but I want it to be evaluated when I execute r.
Is a constructor in Java a method? Is it a special method?
Why? What makes a constructor a method or not a method?
There are examples where it is said that it is a method, such as Constructor in Java :
A constructor is a special method of a class that is automatically called whenever an object of that class is declared.
And examples where the opposite is said as in 12 Rules of Overriding in Java You Should Know :
Because constructors are not methods
...
I'm asking if a constructor is a method.
I'm not asking what a constructor is or what a method is. Nor what are their differences or similarities. These questions may be relevant to whether or not a constructor is a method, but they are not the primary subject of the question.
The question arises from this answer .
It's not okay to derive from classes that weren't meant to be derived from (standard containers). Composition could be used, although I don't see the advantage right now.
Is it so?
What are the drawbacks of deriving from a standard container such as std::set?
Why is composition preferable?
If I have an array in which I want to compose a string and I want to make sure I don't have overflows when I write strings, I can do it like this:
void componer( char compuesto[100], const char* cadena1, const char* cadena2 )
{
sprintf( compuesto, "%.40s--%.40s", cadena1, cadena2 );
}
That way I make sure I never overflow the array compuestoregardless of the length of the strings cadena1and cadena2because it %.40smeans writing a string but at most 40 characters.
But for integers width and precision in formatting printfand similar functions mean the minimum number of characters to print, but more can be printed.
The problem is that this function:
void componeInt( char compuesto[14], int a, int b)
{
sprintf( compuesto, "%d/%d", a, b );
}
it will work fine on architectures where int is 16 bits; because at most an int has 5 decimal places, 6 with the negative sign. And with 14 characters there is always room to store that string.
But on 32-bit architectures I'm going to get an array overflow.
Is there a format parameter to sprintf to restrict the maximum number of characters to print in an integer? If there isn't, how to do it? I don't care if the most significant figures or the least significant figures are written from the partially written number.
I have to write several debugging messages in a program and I want to do it with a macro used like this:
TRAZA( "Hola" << " var=" << var );
This macro must build a character string with the file and the line where it is found in the source code and the message in parentheses, for example, if varit is 7 , it is in the file '/src/a.cpp' and in the line 30 should build this string:
/src/a.cpp:30 - Hello var=7
And you should call the function trace(const char*) with that string.
Note that in the macro parameter I want to be able to put multiple ostreamC++-style chained objects using the operator <<.
The macro was first implemented in the following way:
#define TRAZA( msg ) \
do { \
std::stringstream sstr; \
sstr << __FILE__ << ":" << __LINE__ << " - " << msg; \
traza( sstr.str().c_str() ); \
} while (false)
That works fine except if I have to print a variable called sstr, which would be hidden by the one declared in the macro.
I tried to work around it by using an anonymous object so I don't have to declare a variable:
#define TRAZA( msg ) \
do { \
traza( \
dynamic_cast<std::stringstream&>( \
( std::stringstream() << __FILE__ << ":" << __LINE__ << " - " << msg)) \
.str().c_str() \
); \
} while (false)
But it doesn't write me the name of the file but it writes me the address of the pointer in hexadecimal:
0x401024:30 - Hello var=7
How do I make it so that std::stringstreamanonymous interprets that it should write the character string and not the address that points to that string?
The following program has a complete example:
#include <iostream>
#include <sstream>
#define TRAZA( msg ) \
do { \
traza( \
dynamic_cast<std::stringstream&>( \
( std::stringstream() << __FILE__ << ":" << __LINE__ << " - " << msg)) \
.str().c_str() \
); \
} while (false)
void traza( const char* texto )
{
// En realidad traza() hace otra cosa pero por simplificar
// lo imprimo en la consola.
// Yo no he implementado traza(), no la puedo cambiar y tengo que
// usarla.
std::cout << texto;
}
int main( int argc, char* argv[])
{
int sstr = 7;
TRAZA( "Hola" << " var=" << sstr );
return 0;
}
I have found the following code:
char query[255];
sprintf(query, "SELECT NOMBRE, ");
switch (tipo)
{
case TIPO_1:
sprintf(query, "%s ID", query);
break;
case TIPO_2:
sprintf(query, "%s APELLIDO", query);
break;
}
And what bothers me is the use of queryplaying the role of both the read parameter and the write parameter of sprintf.
The code works fine and does what is expected of it in our compiler and in our tests.
Assuming the size of the array is not exceeded. And that whenever query is used as a read parameter, it is the first parameter after the format parameter and the format starts with "%s".
Is it a correct use? Or is there a situation where it could malfunction?
I am working against a SQLite3 database and I was surprised to be able to insert a line with a reference to another table in which the corresponding row with the foreign key does not exist.
The following minimal example illustrates the situation.
jdura@thinkpad:~/t$ sqlite3
SQLite version 3.8.11.1 2015-07-29 20:00:57
Enter ".help" for usage hints.
Connected to a transient in-memory database.
Use ".open FILENAME" to reopen on a persistent database.
sqlite> CREATE TABLE CLIENTE( NOMBRE TEXT NOT NULL , CIUDAD TEXT,
...> PRIMARY KEY( NOMBRE));
sqlite> CREATE TABLE OTROS_DATOS( NOMBRE TEXT NOT NULL, CANTIDAD INTEGER ,
...> PRIMARY KEY( NOMBRE),
...> FOREIGN KEY(NOMBRE) REFERENCES CLIENTE(NOMBRE)
...> ON DELETE CASCADE
...> ON UPDATE CASCADE DEFERRABLE INITIALLY DEFERRED);
sqlite> INSERT INTO CLIENTE( NOMBRE, CIUDAD) VALUES("JUAN", "CUZCO");
sqlite> SELECT * FROM CLIENTE;
JUAN|CUZCO
sqlite> INSERT INTO OTROS_DATOS( NOMBRE, CANTIDAD) VALUES( "JUAN", 7);
sqlite> SELECT * FROM OTROS_DATOS;
JUAN|7
sqlite> INSERT INTO OTROS_DATOS(NOMBRE, CANTIDAD) VALUES( "NO EXISTE", 27); /*
...> ^^^^ Esperaba fallo por referencia no satisfecha */;
sqlite> SELECT * FROM OTROS_DATOS; /* Pero la línea sí se ha insertado */;
JUAN|7
NO EXISTE|27
sqlite> INSERT INTO OTROS_DATOS(NOMBRE, CANTIDAD) VALUES( "NO EXISTE", 27); /*
...> ^^^^ Aquí sí que falla, al menos los cámpos únicos funcionan */;
SQL error: column NOMBRE is not unique
sqlite>
As I indicated in the comment, I expected that inserting a line in the table OTROS_DATOSwith reference to the client "NO EXISTE", which does not exist in the table, would fail CLIENTE.
How do I have to create the tables so that referential integrity is respected?
I have to instantiate an object of an inner class and it works fine for me inside the class but not outside.
// Fichero Main.java
public class Main {
public static void main( String[] args )
{
Test.Interior interior = new Test.Interior(); // Esta línea falla
Test.InteriorStatic interiorStatic = new Test.InteriorStatic();
}
}
// Fichero Test.java
public class Test {
public class Interior {
}
public static class InteriorStatic {
}
public void algo()
{
Test.Interior interior = new Test.Interior();
}
}
The line in which I indicate that it fails gives the following compilation error:
Main.java:15: error: an enclosing instance that contains Test.Interior is required
But exactly the same line within algo()itself works fine. And as much Testas they Test.Interiorare public.
If I make the class Test.Interioris staticit does not give the error, but it is a class that I have no possibility to modify in the production code.
I need to check that a date entered in a form is valid.
I have the date separated into year, day and month. And I'm not interested in checking dates before 1900.
I am doing it with this code:
public class Fecha
{
int año;
int mes; // 1 a 12
int dia; // 1 a 31
}
static int[] diasMes= {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
/**
* Comprueba si la fecha es correcta. Se comprueban solo fechas de 1900
* o posteriores.
* @param fecha La fecha a comprobar.
* @return true si la fecha es correcta, false en otro caso.
* @throws IllegalArgumentException si el año es menor que 1900.
*/
boolean validaFecha( Fecha fecha)
{
if ( fecha.año < 1900 ) {
throw new IllegalArgumentException(
"Solo se comprueban fechas del año 1900 o posterior");
}
if ( fecha.mes<1 || fecha.mes>12 )
return false;
// Para febrero y bisiesto el limite es 29
if ( fecha.mes==2 && fecha.año%4==0 )
return fecha.dia>=1 && fecha.dia<=29;
return fecha.dia>=0 && fecha.dia<=diasMes[fecha.mes-1];
}
The class FechaI use is somewhat more complicated, it has its setter, getter and other things, but those details are irrelevant to this question. The question is about the method validaFecha.
Is the implementation correct? Is there a way to do it with the standard libraries?
Yesterday I wrote by mistake #include "iostream"and I haven't realized it because the compiler didn't give an error. Later I realized and I was surprised.
I have tried with this small program :
// Fichero a.h
#ifndef A_H
#define A_H
#endif // A_H
// Fichero a.cpp
#include <iostream>
#include "a.h"
int main(void) {
std::cout << "Hola Mundo" << std::endl;
return 0;
}
Which works as expected.
It also works if I put the two includes with double quotes " ", this has surprised me.
If I put the two includes with minor major < >it gives me a compilation error, which is what I expect:
a.cpp:2:15: fatal error: ah: No such file or directory
I thought that :
#include < >it is for system headers or libraries.#include " "it is for programmer headers.But clearly not because iostream can also be included with " ".
Can I just include it all with " "?
Or could it have some adverse effect doing everything with " "? And if so, what criteria to use to decide whether each include is done with " "or < >?
In the project assigned to me, they use a very primitive set of makefiles in which each .o object depends only on the .cpp from which it is generated.
Which forces us to do a make cleanevery time we have to compile something with the consequent loss of time because it takes a couple of minutes to compile everything.
Simplifying it to the essence of the problem, the makefile I work with is this:
CPP=g++
CPPFLAGS = -fPIC -Wall
INCLUDES =
.SUFFIXES: .cpp
.cpp.o :
$(CPP) $(CPPFLAGS) $(INCLUDES) -o $@ -c $<
OBJETOS =\
a.o\
b.o\
%.o : %.cpp
$(CPP) $(CPPFLAGS) $(INCLUDES) -o $(@F) -c $<
ejecutable: $(OBJETOS)
$(CPP) $(LDFLAGS) $(OBJETOS) -o ejecutable
all : ejecutable
clean :
-rm -f *.o
-rm -f ejecutable
And the .cpp and .h files would be the following:
// Fichero a.h
#ifndef _A_H_
#define _A_H_
#endif // _A_H_
// Fichero b.h
#ifndef _B_H_
#define _B_H_
#endif // _B_H_
// Fichero comun.h
#include "comun.h"
#ifndef _COMUN_H_
#define _COMUN_H_
#endif // _COMUN_H_
// Fichero a.cpp
#include "a.h"
// Fichero b.cpp
#include "a.h"
#include "b.h"
int main(void) {
return 0;
}
It's easy to see that with the above makefile a change to ah would not cause a recompile on do make, which is why we need to do 'make clean' first.
Making a correct makefile for the example above is easy:
CPP=g++
CPPFLAGS = -fPIC -Wall
INCLUDES =
.SUFFIXES: .cpp
.cpp.o :
$(CPP) $(CPPFLAGS) $(INCLUDES) -o $@ -c $<
OBJETOS =\
a.o\
b.o\
ejecutable: $(OBJETOS)
$(CPP) $(LDFLAGS) $(OBJETOS) -o ejecutable
a.o : a.cpp a.h comun.h
b.o : b.cpp a.h b.h comun.h
%.o : %.cpp
$(CPP) $(CPPFLAGS) $(INCLUDES) -o $(@F) -c $<
all : ejecutable
clean :
-rm -f *.o
-rm -f ejecutable
But this improved makefile has two problems:
#include, you have to modify the makefile.How can I make the makefile calculate the dependencies every time?
I'm confused about the precedence of the direct assignment operator and the ternary conditional operator:
#include<stdio.h>
int main(void)
{
int j, k;
j = k = 0;
(1 ? j : k) = 1; // primero
printf("%d %d\n", j, k);
j = k = 0;
1 ? j : k = 1; // segundo
printf("%d %d\n", j, k);
return 0;
}
I expected the output to be:
1 0
1 0
But it turns out to be:
1 0
0 0
And I get this build notice:
main.cpp:20: warning: statement has no effect
Which is about the commented line as second.
Since the assignment operator has lower precedence than the ternary conditional operator, I expected the first and second commented lines to be equivalent. But it turns out not.
I have tried with g++ (Ubuntu 5.2.1-22ubuntu2) 5.2.1 20151010
This question is a translation of the English original that I asked myself.
I have seen in a comment on the English site that these two lines do different things:
Object obj;
Object obj{};
It's clear to me that the first declares a variable of class Object and builds it with the parameterless constructor.
But what does the second do? In more than 10 years programming in C++ I have never seen it, is it something new from c++11 or 14?
Both Java and C++ have references.
Are they practically the same or are they significantly different?
Java has no pointers. So are references in Java like pointers in C++?
Are there code structures around references and pointers that are the same in the two languages but do different things?
Here is a very peculiar piece of C++ code. For some strange reason sorting the data miraculously causes the code to run 3 times faster.
#include <algorithm>
#include <ctime>
#include <iostream>
int main()
{
// Generate data
const unsigned arraySize = 32768;
int data[arraySize];
for (unsigned c = 0; c < arraySize; ++c)
data[c] = std::rand() % 256;
// !!! Con esto el siguiente bucle se ejecuta más rápido.
std::sort(data, data + arraySize);
// Test
clock_t start = clock();
long long sum = 0;
for (unsigned i = 0; i < 100000; ++i)
{
// Primary loop
for (unsigned c = 0; c < arraySize; ++c)
{
if (data[c] >= 128)
sum += data[c];
}
}
double elapsedTime = static_cast<double>(clock() - start) / CLOCKS_PER_SEC;
std::cout << elapsedTime << std::endl;
std::cout << "sum = " << sum << std::endl;
}
std::sort(data, data + arraySize);, the code runs in 12.41 seconds.At first I thought it might be a language or compiler anomaly. So I tried in Java :
import java.util.Arrays;
import java.util.Random;
public class Main
{
public static void main(String[] args)
{
// Generate data
int arraySize = 32768;
int data[] = new int[arraySize];
Random rnd = new Random(0);
for (int c = 0; c < arraySize; ++c)
data[c] = rnd.nextInt() % 256;
// !!! Con esto el siguiente bucle se ejecuta más rápido.
Arrays.sort(data);
// Test
long start = System.nanoTime();
long sum = 0;
for (int i = 0; i < 100000; ++i)
{
// Primary loop
for (int c = 0; c < arraySize; ++c)
{
if (data[c] >= 128)
sum += data[c];
}
}
System.out.println((System.nanoTime() - start) / 1000000000.0);
System.out.println("sum = " + sum);
}
}
Which gives similar but less extreme results. 10.7 and 6.2 seconds.
At first I thought it might have to do with sorting the data getting it into the cache, and then I realized that the data had just been generated with what should already be in the cache before sorting.
This is a question translated from the original in English and adapted to the results it gives on my computer: Why is processing a sorted array faster than an unsorted array? by GManNickG