CODEWITHSUNDEEP
c++c++11c-preprocessortemplate-meta-programmingpreprocessor-meta-program
Mohit
Mohit

Reputation: 2100

How to get c++ function caller name at preprocessing stage

I have to use a macro PERF_INSTRUMENT from a library. PERF_INSTRUMENT expects a user provided c-style string as a function name to print the location of this instrument point.

But, I don't want to write the function name everytime I use PERF_INSTRUMENT instead I want to call it with __func__ so that function name is automatically included in the perf log.

But when I use __func__ it actually returns operator() because the __func__ is embedded inside the lambda function.

Is their any way by which I can pass the main() function name to the PERF_INSTRUMENT macro.

#include <cstdio>
#include <cassert> 
#include <type_traits> 

using namespace std;

namespace /* anonymous */
{
    template< typename T >
    struct Is_Const_Char_Array
      : std::is_same< std::remove_reference_t< T >,
                      char const[ std::extent< std::remove_reference_t< T > >::value ] >
    {};

    template< typename T >
    struct Is_C_String_Literal
      : Is_Const_Char_Array< T >
    {};
}

#define PERF_INSTRUMENT(name)  auto instObj = [] { static_assert( Is_C_String_Literal< decltype( name ) >::value, "input argument must be a c-string literal" ); /* Some other Logic*/ printf(name);return 1; }()


// <------------------ MY CODE -------------------> //

int main(){
    PERF_INSTRUMENT("main"); // <-- this works fine
    PERF_INSTRUMENT(__func__); // <-- this prints operator()
    // PERF_INSTRUMENT(__builtin_FUNCTION());
}

Please Note that I can only change the code below the MY CODE line

Upvotes: 3

Views: 1307

Answers (2)

max66
max66

Reputation: 66190

Is their any way by which I can pass the main function name to the PERF_INSTRUMENT macro.

You can pass "name" as argument to the lambda itself.

Something as

#define PERF_INSTRUMENT(name) \
    auto instObj = [](char const * str) \ // <-- receive an argument
       { static_assert( Is_C_String_Literal< decltype( name ) >::value, \
                       "input argument must be a c-string literal" );\
         /* Some other Logic*/ \
         printf(str); \  // <-- print the argument received, not directly name
         return 1;\
       }(name)
//.......^^^^   pass name as argument

Bonus Off Topic proposal: to detect is an object is a C-string literal, i propose an alternative way

template <typename T>
constexpr std::false_type islHelper (T, long);

template <typename T, std::size_t N>
constexpr std::true_type islHelper (T const(&)[N], int);

template <typename T>
using isStringLiteral = decltype(islHelper(std::declval<T>(), 0));

In static_assert() become

static_assert( isStringLiteral<decltype(name)>::value,
               "input argument must be a c-string literal" );

Upvotes: 5

molbdnilo
molbdnilo

Reputation: 66371

Since the assert is fundamentally flawed – it accepts any const char array – wrapping the macro in another macro should work.
Something like this:

#define PERF_FUNCTION do { \
    const char name[] = __func__; \
    PERF_INSTRUMENT(name); \
} while(0)

Upvotes: 0

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