检查表达式是否为“ constexpr”

Question

C++17 adds constexpr if ( to choose if to compile a statement if the condition is a constexpr ) http://en.cppreference.com/w/cpp/language/if

Is there a trick to emulate a limited form of this construct in C++11 ?

If would need the following construct in a macro:

  #define ALLOCATE(x) if ({x is a constant}) allocate_n<x>() else allocate(x)

Answer 1

Supposing that you're using GCC or Clang, you can use the __builtin_constant_p() extension:

#define ALLOCATE(x) \
    __builtin_constant_p(x) \
        ? allocate_n<__builtin_constant_p(x) ? x : 0>() \
        : allocate(x)

See it live on Coliru

Answer 2

Is there a trick to emulate a limited form of this construct in C++11 ?

The best I can imagine is substitute

if constexpr ( cond ) statment-1 else statement-2;

with

foo<cond>( /* ? */ );

where foo() is defined as follows

template <bool>
void foo (/* ? */);

template <>
void foo<true> (/* ? */)
 { /* statement-1 */ }

template <>
void foo<false> (/* ? */)
 { /* statement-2 */ }