static_assert

Syntax

  • static_assert( bool_constexpr, message )
  • static_assert( bool_constexpr ) /* Since C++17 */

Parameters

ParameterDetails
bool_constexprExpression to check
messageMessage to print when bool_constexpr is false

Remarks

Unlike runtime assertions, static assertions are checked at compile-time and are also enforced when compiling optimized builds.

static_assert

Assertations mean that a condition should be checked and if it's false, it's an error. For static_assert(), this is done compile-time.

template<typename T>
T mul10(const T t)
{
    static_assert( std::is_integral<T>::value, "mul10() only works for integral types" );
    return (t << 3) + (t << 1);
}

A static_assert() has a mandatory first parameter, the condition, that is a bool constexpr. It might have a second parameter, the message, that is a string literal. From C++17, the second parameter is optional; before that, it's mandatory.

C++17
template<typename T>
T mul10(const T t)
{
    static_assert(std::is_integral<T>::value);
    return (t << 3) + (t << 1);
}

It is used when:

  • In general, a verification at compile-time is required on some type on constexpr value
  • A template function needs to verify certain properties of a type passed to it
  • One wants to write test cases for:
    • template metafunctions
    • constexpr functions
    • macro metaprogramming
  • Certain defines are required (for ex., C++ version)
  • Porting legacy code, assertations on sizeof(T) (e.g., 32-bit int)
  • Certain compiler features are required for the program to work (packing, empty base class optimization, etc.)

Note that static_assert() does not participate in SFINAE: thus, when additional overloads / specializations are possible, one should not use it instead of template metaprogramming techniques (like std::enable_if<>). It might be used in template code when the expected overload / specialization is already found, but further verifications are required. In such cases, it might provide more concrete error message(s) than relying on SFINAE for this.



2016-07-26
2017-06-01
C++ Pedia
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