Variadic Macro: cannot pass objects of non-trivially-copyable type through '…'

问题

I am trying to write a macro for logging mechanism. I wrote a variadic macro but it does not work with std::string. The code looks like the following:

#include <stdio.h>
#include <string>


#define LOG_NOTE(m, ...) printf(m, ##__VA_ARGS__)

int main()
{
    std::string foo = "random string";
    int bar = 5;
    LOG_NOTE("%s %d %s", "Hello World", bar, foo);

    return 0;
}

If I would call the macro like following, I would not get any error.

LOG_NOTE("%s %d %s", "Hello World", bar, "random string");

Compiler Output:

In function 'int main()': 5:49: error: cannot pass objects of non-trivially-copyable type 'std::string {aka class std::basic_string}' through '...' 11:5: note: in expansion of macro 'LOG_NOTE'

回答1:

I wrote a variadic macro

Don't. Use a variadic template function.

The actual problem you have is that you're trying to pass a C++ object (std::string) through a C API (printf). This is not possible.

You'd need some mechanism for conversion, for example:

#include <stdio.h>
#include <string>

template<class T>
decltype(auto) convert_for_log_note(T const& x)
{
    return x;
}

decltype(auto) convert_for_log_note(std::string const& x)
{
    return x.c_str();
}


template<class...Args> 
void LOG_NOTE(const char* format, Args&&...args)
{
    printf(format, convert_for_log_note(args)...);
}

int main()
{
    std::string foo = "random string";
    int bar = 5;
    LOG_NOTE("%s %d %s\n", "Hello World", bar, foo);

    return 0;
}

Example output:

Hello World 5 random string

http://coliru.stacked-crooked.com/a/beb3431114833860

Update:

For C++11 you'll need to spell out the return types by hand:

#include <stdio.h>
#include <string>

template<class T>
T const& convert_for_log_note(T const& x)
{
    return x;
}

const char* convert_for_log_note(std::string const& x)
{
    return x.c_str();
}


template<class...Args> 
void LOG_NOTE(const char* format, Args&&...args)
{
    printf(format, convert_for_log_note(args)...);
}

int main()
{
    std::string foo = "random string";
    int bar = 5;
    LOG_NOTE("%s %d %s\n", "Hello World", bar, foo);

    return 0;
}

回答2:

The issue here is not the variadic macro, but the call to printf. Have a look at the documentation: the format specifier "%s" corresponds to char*, not std::string. printf can only handle primitive builtin types. You can change you invocation to

LOG_NOTE("%s %d %s", "Hello World", bar, foo.c_str());

to fix this.

回答3:

You cannot pass object to printf, so you have currently to use

LOG_NOTE("%s %d %s", "Hello World", bar, foo.c_str());

If you don't need formatting, and just write every argument separated with space, you might simply use variadic template instead of MACRO:

template <typename ... Ts>
void LOG_NOTE(const Ts&...args)
{
    const char* sep = "";
    (((std::cout << sep << args), sep = " "), ...); // C++17 folding expression
    // C++11/C++14 version are more verbose:
    // int dummy[] = {0, ((std::cout << sep << args), (sep = " "), 0)...};
    // static_cast<void>(dummy); // avoid warning for unused variable
}

int main()
{
    std::string foo = "random string";
    int bar = 5;
    LOG_NOTE("Hello World", bar, foo);
}

Demo

回答4:

I could not get @richardhodges nice solution to work in any of the C++11 compilers I tried. However, the following works with gcc -std=c++11:

#include <stdio.h>
#include <string>

template<class T>
T convert_for_log_note(T const& x)
{
    return x;
}

inline const char* convert_for_log_note(std::string const& x)
{
    return x.c_str();
}


template<class...Args> 
void LOG_NOTE(const char* format, Args&&...args)
{
    printf(format, convert_for_log_note(args)...);
}

int main()
{
    std::string foo = "random string";
    int bar = 5;
    LOG_NOTE("%s %d %s\n", "Hello World", bar, foo);

    return 0;
}

The inline keyword is necessary with the above solution for the Arduino C++ compiler, whereas other g++ compilers do not require it (the ones I have tried, anyways). Without this keyword, the Arduino code compiles, but the linker complains about multiple definitions.

来源：https://stackoverflow.com/questions/51856415/variadic-macro-cannot-pass-objects-of-non-trivially-copyable-type-through