How to print result of a compile-time calculation in C++?

问题

I've wrote several constexpr functions and use them in static_asserts to control some resource limits. But I'd like to not only enforce compile-time predicate but also to see the actual values calculated during normal compilation process or at least when assertion fails.

There are ways to print string messages during compilation, but what's about printing results of constexpr computations?

回答1:

Here is some code that exploits gcc's diagnostic messages to print values of interest after an assert message. To find the values of interest, you just need to search the error string for T x =:

#include <string>

template <class T, T x, class F>
void transparent(F f) { f(); }


template <bool B>
constexpr void my_assert() { 
    static_assert(B, "oh no");
}

template <int X>
void f() {
    transparent<int, X+7>([]{
        transparent<long, X*X*X>([]{
            my_assert<X+10==-89>(); });});
}

int main() {
//    f<3>();
    f<4>();
//    f<-99>();
}

Here is the error that it got me:

g++ h.cpp -std=c++11 h.cpp: In instantiation of ‘constexpr void my_assert() [with bool B = false]’: h.cpp:16:34: required from ‘f() [with int X = 4]::__lambda0::__lambda1’ h.cpp:15:35: required from ‘struct f() [with int X = 4]::__lambda0::__lambda1’ h.cpp:16:38: required from ‘f() [with int X = 4]::__lambda0’ h.cpp:14:28: required from ‘struct f() [with int X = 4]::__lambda0’ h.cpp:16:41: required from ‘void f() [with int X = 4]’ h.cpp:21:10: required from here h.cpp:9:5: error: static assertion failed: oh no static_assert(B, "oh no"); ^ h.cpp:4:6: error: ‘void transparent(F) [with T = long int; T x = 64l; F = f() [with int X = 4]::__lambda0::__lambda1]’, declared using local type ‘f() [with int X = 4]::__lambda0::__lambda1’, is used but never defined [-fpermissive] void transparent(F f) { f(); } ^ h.cpp:4:6: error: ‘void transparent(F) [with T = int; T x = 11; F = f() [with int X = 4]::__lambda0]’, declared using local type ‘f() [with int X = 4]::__lambda0’, is used but never defined [-fpermissive]

Note that bolded parts

回答2:

My VC++ code that print, during compilation, the value of as many compile time constants as you like (e.g. sizeof structures) and continue compiling without error:

// cpptest.cpp : Defines the entry point for the console application.
//

#include "stdafx.h"
//#define VALUE_TO_STRING2(x) #x
//#define VALUE_TO_STRING(x) VALUE_TO_STRING2(x)


#define TO_STRING(x) #x
#define FUNC_TEMPLATE_MSG(x,y) "[" x "]""["TO_STRING(y)"]"

template<unsigned int N,unsigned int M> 
int printN()
{ 
#pragma message(FUNC_TEMPLATE_MSG(__FUNCSIG__ ,1))

    return 0;
};



struct X {
    char a[20];
    int b;
};
struct Y {
    char a[210];
    int b;
};

int _tmain(int argc, _TCHAR* argv[])
{
printN<sizeof(X),__COUNTER__>();
printN<sizeof(Y),__COUNTER__>();
//..as many compile time constants as you like
}

Sample output produced by VC++2010. The target value is the first function template parameter value (0x18 and 0xd8 in the example) which VC++ oddly chose to output as hexadecimal value!!

1>------ Build started: Project: cpptest, Configuration: Release Win32 ------
1>  cpptest.cpp
1>  [int __cdecl printN<0x18,0x0>(void)][1]
1>  [int __cdecl printN<0xd8,0x1>(void)][1]
1>  Generating code
1>  Finished generating code
1>  cpptest.vcxproj -> c:\work\cpptest\Release\cpptest.exe
========== Build: 1 succeeded, 0 failed, 0 up-to-date, 0 skipped ==========

The major technique used here is that a during each function template instantiation, the #pragma message() directive is invoked once. The Microsoft specific macro __FUNCSIG__ can display the signature of the containing function and thus the integer value used in each specific instantiation of the function template. Giving COUNTER as the second template parameter is to make sure 2 integers of the same value are still considered different. The 1 in the #pragma directive is of no use here but can be used as an identifier in case we have more than 1 such directive and the output window is full of messy messages.

回答3:

This is based on the solution posted by @tohava:

If you need to print constexpr values without static_assert(), this works for the GCC compiler with the -Wunused flag:

// before c++17:
template <typename T, T val>
constexpr void static_print() {
    #if !defined(__GNUC__) || defined(__clang__)
        int static_print_is_implemented_only_for_gcc = 0;
    #else
        int unused = 0;
    #endif
};

// for c++17 and higher:
template <auto val>
constexpr void static_print() {
    #if !defined(__GNUC__) || defined(__clang__)
        int static_print_is_implemented_only_for_gcc = 0;
    #else
        int unused = 0;
    #endif
};

int main() {
    constexpr int i = 13;
    // for c++17 and higher:
    static_print<i>();
    // before c++17:
    static_print<int, i>();
}

Output:

$ g++ -std=c++17 main.cpp -Wall && ./a
main.cpp: In instantiation of 'constexpr void static_print() [with auto val = 13]':
main.cpp:23:21:   required from here
main.cpp:17:7: warning: unused variable 'unused' [-Wunused-variable]
   int unused = 0;
       ^~~~~~
main.cpp: In instantiation of 'constexpr void static_print() [with T = int; T val = 13]':
main.cpp:24:26:   required from here
main.cpp:8:7: warning: unused variable 'unused' [-Wunused-variable]
   int unused = 0;
       ^~~~~~

The important part is the val = 13.

Play with this example online at https://godbolt.org/z/Cdb-At

Unfortunately, the Clang compiler doesn't include backtrace of the warning message, so it doesn't output the value.

来源：https://stackoverflow.com/questions/28852574/how-to-print-result-of-a-compile-time-calculation-in-c

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