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wonderly gonzalez
Asked: 2020-08-23 21:26:16 +0800 CST 2020-08-23 21:26:16 +0800 CST

C++ wrong result

  • 772

I have a problem in c++ I am trying to do a simple calculation using OOP but the result is wrong and not close to what it should be:

#include <iostream>

using namespace std;

class productos{

public:

productos(int x,int y){
x=n;
y=m;
cout<<x*y;
}
int producto1(){

    return n*m;//Usando esta funcion tampoco arroja el resultado
}



private:
int m; 
int n;
int mil;




};
int main()
{
    productos dos(2,2500);



return 0;
}

The result is:

-1074975852-1074975852

It also gave me an error initializing:

int m=2500;

I am a beginner in c++ and I have other practically identical console programs, maybe with more functions but basically the same.

What did I do wrong? and how do i fix it?

PS: I already tried with sublime text codeblocks and an online compiler and the result is the same.

c++

3 Answers

  1. Best Answer

    Problem with uninitialized variables

    You are making a very common mistake and that is to make use of a variable without having been previously set to a value.

    void productos(int x, int y) {
  x = n;
  y = m;
  std::cout << x * y;
}

    As you can see here you are assigning y to the values ​​of xand , overriding the values ​​you have passed to the constructor, but these have no values ​​initially, so their initial content is indeterminate.ynm

    private:
 int m;
 int n;
 int mil;

    Solution making correct use of the class

    If I reorder the code to do what you want, this would be the result:

    #include <iostream>

class productos {
 public:

  productos(int x, int y) {
    n = x;
    m = y;
    std::cout << "En el constructor: " << x * y << std::endl;
 }

  int producto1() {
    return n * m;
  }

 private:
  int m;
  int n;
  int mil;
};

int main() {
  /* Creo una instancia de productos pasando al constructor (2, 2500) */
  productos producto (2, 2500);
  /* Uso el método producto1 para calcular el resultado de la multiplicación */
  std::cout << "El resultado del producto es: " << producto.producto1() << std::endl;
  return 0;
}

    The result of the execution is:

    En el constructor: 5000
El resultado del producto es: 5000

    Error analysis with valgrind

    Valgrind results from your code:

    ==30015== Memcheck, a memory error detector
==30015== Copyright (C) 2002-2013, and GNU GPL'd, by Julian Seward et al.
==30015== Using Valgrind-3.10.1 and LibVEX; rerun with -h for copyright info
==30015== Command: ./pruebas_stack
==30015== 
==30015== Conditional jump or move depends on uninitialised value(s)
==30015==    at 0x4EBFCDE: std::ostreambuf_iterator<char, std::char_traits<char> > std::num_put<char, std::ostreambuf_iterator<char, std::char_traits<char> > >::_M_insert_int<long>(std::ostreambuf_iterator<char, std::char_traits<char> >, std::ios_base&, char, long) const (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.19)
==30015==    by 0x4EC02BC: std::num_put<char, std::ostreambuf_iterator<char, std::char_traits<char> > >::do_put(std::ostreambuf_iterator<char, std::char_traits<char> >, std::ios_base&, char, long) const (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.19)
==30015==    by 0x4ECC06D: std::ostream& std::ostream::_M_insert<long>(long) (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.19)
==30015==    by 0x4008DB: productos::productos(int, int) (in ./pruebas_stack)
==30015==    by 0x40084A: main (in ./pruebas_stack)
==30015== 
==30015== Use of uninitialised value of size 8
==30015==    at 0x4EBFBC3: ??? (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.19)
==30015==    by 0x4EBFD05: std::ostreambuf_iterator<char, std::char_traits<char> > std::num_put<char, std::ostreambuf_iterator<char, std::char_traits<char> > >::_M_insert_int<long>(std::ostreambuf_iterator<char, std::char_traits<char> >, std::ios_base&, char, long) const (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.19)
==30015==    by 0x4EC02BC: std::num_put<char, std::ostreambuf_iterator<char, std::char_traits<char> > >::do_put(std::ostreambuf_iterator<char, std::char_traits<char> >, std::ios_base&, char, long) const (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.19)
==30015==    by 0x4ECC06D: std::ostream& std::ostream::_M_insert<long>(long) (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.19)
==30015==    by 0x4008DB: productos::productos(int, int) (in ./pruebas_stack)
==30015==    by 0x40084A: main (in ./pruebas_stack)
==30015== 
==30015== Conditional jump or move depends on uninitialised value(s)
==30015==    at 0x4EBFBCF: ??? (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.19)
==30015==    by 0x4EBFD05: std::ostreambuf_iterator<char, std::char_traits<char> > std::num_put<char, std::ostreambuf_iterator<char, std::char_traits<char> > >::_M_insert_int<long>(std::ostreambuf_iterator<char, std::char_traits<char> >, std::ios_base&, char, long) const (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.19)
==30015==    by 0x4EC02BC: std::num_put<char, std::ostreambuf_iterator<char, std::char_traits<char> > >::do_put(std::ostreambuf_iterator<char, std::char_traits<char> >, std::ios_base&, char, long) const (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.19)
==30015==    by 0x4ECC06D: std::ostream& std::ostream::_M_insert<long>(long) (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.19)
==30015==    by 0x4008DB: productos::productos(int, int) (in ./pruebas_stack)
==30015==    by 0x40084A: main (in ./pruebas_stack)
==30015== 
==30015== Conditional jump or move depends on uninitialised value(s)
==30015==    at 0x4EBFD33: std::ostreambuf_iterator<char, std::char_traits<char> > std::num_put<char, std::ostreambuf_iterator<char, std::char_traits<char> > >::_M_insert_int<long>(std::ostreambuf_iterator<char, std::char_traits<char> >, std::ios_base&, char, long) const (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.19)
==30015==    by 0x4EC02BC: std::num_put<char, std::ostreambuf_iterator<char, std::char_traits<char> > >::do_put(std::ostreambuf_iterator<char, std::char_traits<char> >, std::ios_base&, char, long) const (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.19)
==30015==    by 0x4ECC06D: std::ostream& std::ostream::_M_insert<long>(long) (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.19)
==30015==    by 0x4008DB: productos::productos(int, int) (in ./pruebas_stack)
==30015==    by 0x40084A: main (in ./pruebas_stack)
==30015== 
-251679360
==30015== 
==30015== HEAP SUMMARY:
==30015==     in use at exit: 0 bytes in 0 blocks
==30015==   total heap usage: 0 allocs, 0 frees, 0 bytes allocated
==30015== 
==30015== All heap blocks were freed -- no leaks are possible
==30015== 
==30015== For counts of detected and suppressed errors, rerun with: -v
==30015== Use --track-origins=yes to see where uninitialised values come from
==30015== ERROR SUMMARY: 20 errors from 4 contexts (suppressed: 0 from 0)

    Where I highlight the meaning of:

    • Use of uninitialized value of size 8 : Use of an uninitialized value of size 8 .
    • Conditional jump or move depends on uninitialized value(s) : Conditional jump depends on an uninitialized value(s) .

    Results of valgrind after correcting the problem:

    ==29964== Memcheck, a memory error detector
==29964== Copyright (C) 2002-2013, and GNU GPL'd, by Julian Seward et al.
==29964== Using Valgrind-3.10.1 and LibVEX; rerun with -h for copyright info
==29964== Command: ./pruebas_stack
==29964== 
5000
==29964== 
==29964== HEAP SUMMARY:
==29964==     in use at exit: 0 bytes in 0 blocks
==29964==   total heap usage: 0 allocs, 0 frees, 0 bytes allocated
==29964== 
==29964== All heap blocks were freed -- no leaks are possible
==29964== 
==29964== For counts of detected and suppressed errors, rerun with: -v
==29964== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)
    • 2

  2. The problem is in the assignment inside productos(int x, int y)since you are assigning to these variables inside the function, maybe you reversed the order of the variables; you have 2 options:

    1. In the function products you receive the variables n and mproductos(int n; int m)
    2. In the products function you leave it as you have it (receiving x and y) but you modify the assignment n = xy m = yand printcout<<n*m

    I did it with way 1: 

    #include <iostream>

class productos{

    public:

    productos(int n,int m){
        x=n;
        y=m;
        std::cout<< x*y;
    }

    private:
    int x;
    int y;
};

int main()
{
    productos(2,2500);
    return 0;
}

    The values ​​it was throwing at you -1074975852-1074975852is because it was taking values ​​from memory when you were doing assignments x = nand n had no value.

    • 1

  3. To avoid variable initialization errors, which I am not going to detail because you already have other answers in which they comment on the problem, it is preferable to initialize all the member variables that is possible in the initialization section within the constructor, let me explain:

    productos(int x, int y)
  : n(x), m(y)
{
  std::cout << "En el constructor: " << x * y << std::endl;
}

    Why? Two basic reasons:

    • In the case of initializing classes you avoid redundant operations:

      struct Test
{
  std::string a;

  // Aqui se hace
  // Llamada al constructor de std::string
  // Llamada al operador de asignación std::string = std::string
  Test(std::string const& var)
  {
    a = var;
  }

  // Aqui se hace
  // Llamada al constructor copia std::string(std::string const&)
  Test(std::string const& var)
    : a(var)
  { }
};

    • As a general rule, avoid silly mistakes:

      struct Test
{
  int a;

  // Error de compilacion
  Test(int b)
    : b(a)
  { }

  // ok
  Test(int b)
    : a(b)
  { }

  // ok. Equivalente a this->a = a
  Test(int a)
    : a(a)
  { }
};
    • 0