template-based compile time assert with custom messages can only be compiled in some of the compilers

Question

This code demos a compile time assert using template. I found it can only be compiled by g++ (4.4.7) with the following cmd line.

$ g++ -std=c++98 a.cpp -o a

Nether icc (13.0.1) nor visual c++ (14.00.50727.762 for 80x86) can compile it. For icc, it generate error msg like this

$ icpc a.cpp -o a
a.cpp(13): error: non-integral operation not allowed in nontype template argument
      COMPILE_TIME_ASSERT(true && "err msg");
      ^

a.cpp(13): error: class "CompileTimeAssert<<error-constant>>" has no member "Check"
      COMPILE_TIME_ASSERT(true && "err msg");
      ^

compilation aborted for a.cpp (code 2)

However I found assertions like true && "err msg" are widely used in run-time assert as Add custom messages in assert?

Questions are

Can this be solved without modifying the code, only with proper compile options?
If can't, any alternative methods of compile time assert with custom messages?

Demo code show as follows.

#include <iostream>

template<bool B> class CompileTimeAssert { };
template<> class CompileTimeAssert<true> {
 public:
  static inline void Check() { }
};

#define COMPILE_TIME_ASSERT(b) CompileTimeAssert<(b)>::Check()

int main()
{
    COMPILE_TIME_ASSERT(true && "err msg");
    std::cout<<(true && "err msg")<<std::endl;
    return 0;
}

@YochaiTimmer NO, I just want to show g++ can compile the code even with that option — kangshiyin, Jul 25 '13 at 14:27
For non-type template arguments, only integral arguments are legal. — dans3itz, Jul 25 '13 at 14:27
@Eric so why don't you just use static_assert (C++0x & C++11) — Yochai Timmer, Jul 25 '13 at 14:29
Actually, it appears that boost's option doesn't even support the message w/o a minimum target of 0x (early adoption/03 draft). — dans3itz, Jul 25 '13 at 14:36
I have never used that approach for a static assert (I employed the `typedef char assert_XXX[(condition?1:-1)];`), but you could consider changing the macro so that the string is passed to the function: `COMPILE_TIME_ASSERT(condition,"Message")` would expand to `CompileTimeAssert<(condition)>::Check("Message")` which after adapting the signature will probably be easier to digest for all compilers — David Rodríguez - dribeas, Jul 25 '13 at 14:54

score 0 · Answer 1 · answered Oct 27 '13 at 20:00

Answer to the first question is "No". An argument you pass to the template is so called "non-type template argument". According to the standard, such arguments must be:

constant expressions, addresses of functions or objects with external linkage, or addresses of static class members.

An expression like true && "err msg" strictly speaking cannot be determined at compile time. That's why it can be used with run-time assertions but not in compile-time ones. G++ demonstrates here non-standard behaviour.

As an answer to the second question, I propose the following pattern:

#define STATIC_ASSERT(e,m) extern char (*__static_assert_failed(void)) [ 1 - 2*!(e) ]

__static_assert_failed here is a pointer to external function returning an array of chars. The array has size of 1 - 2*!(e) which became negative if e is false causing compile-time error.

template-based compile time assert with custom messages can only be compiled in some of the compilers

1 Answers1