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Anonymous Coward
Asked: 2020-01-21 04:12:25 +0800 CST 2020-01-21 04:12:25 +0800 CST

How to write a const char* to an anonymous std::stringstream?

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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;
}
c++

1 Answers

  1. Best Answer

    The problem occurs because the insert operator does not have a specific overload for const char*. Instead it has a generic entry for pointers ostream& operator<< (void* val). If no checks are made in said function, the only thing that can be obtained from a generic pointer is the address to which it points. In later versions, at least from C++11 onwards, the problem is not reproduced.

    One possible option is to use the method writeof ostream. This method is overloaded and, unlike the insert operator, includes an overload specific to const char*. In this way ambiguities are avoided and the chain can be dumped without problems.

    Another interesting detail is that, like the insertion operator, the function writereturns a reference to the stream and this allows calls to be concatenated.

    #define TRAZA( msg ) \
 do { \
   traza( \
       static_cast<std::stringstream&>( \
         ( std::stringstream().write(__FILE__,strlen(__FILE__)) << ":" << __LINE__ << " - " << msg)) \
         .str().c_str() \
        );  \
 } while (false)

    By the way, note that I have changed dynamic_castto static_cast. You really know that the object is of type std::stringstream, so a static cast is faster since it doesn't perform additional checks to see if the conversion can be done.

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