Static Assertion

 

C++

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Regular expressions library (C++11)

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Thread support library (C++11)

 

C++ language

 

Declarations

Overview declaration syntax decl-specifier-seq declarator Specifiers typedef inline function specifier virtual function specifier explicit function specifier friend constexpr(C++11) storage class specifiers class union enum decltype (C++11) auto (C++11) alignas (C++11) const/volatile attributes (C++11) Declarators reference pointer array

Block declarations simple-declaration alias declaration(C++11) namespace alias definition using-declaration using-directive static_assert declaration (C++11) asm-definition opaque enum declaration(C++11) Other declarations namespace definition function declaration class template declaration function template declaration explicit template instantiation(C++11) explicit template specialization linkage specification attribute declaration (C++11) empty declaration

 

Performs compile-time assertion checking

Syntax

static_assert ( bool_constexpr , message )		(since C++11)

Explanation

bool_constexpr	-	a constant expression that is contextually convertible to bool
message	-	string literal that will appear as compiler error if bool_constexpr is false

A static assert declaration may appear at block scope (as a block declaration) and inside a class body (as a member declaration)

Note

Since message has to be a string literal, it cannot contain dynamic information or even a constant expression that is not a string literal itself. Typically, it cannot contain the name of the template type argument.

Example

Run this code

#include <type_traits>
 
template <class T>
void swap( T& a, T& b)
{
    static_assert(std::is_copy_constructible<T>::value,
                  "Swap requires copying");
    static_assert(noexcept(std::is_nothrow_move_constructible<T>::value
                        && std::is_nothrow_move_assignable<T>::value),
                  "Swap may throw");
    auto c = b;
    b = a;
    a = c;
}
 
template <class T>
struct data_structure
{
    static_assert(std::is_default_constructible<T>::value,
                  "Data Structure requires default-constructible elements");
};
 
struct no_copy
{
    no_copy ( const no_copy& ) = delete;
    no_copy () = default;
};
 
struct no_default
{
    no_default () = delete;
};
 
int main()
{
    int a, b;
    swap(a, b);
 
    no_copy nc_a, nc_b;
    swap(nc_a, nc_b); // 1
 
    data_structure<int> ds_ok;
    data_structure<no_default> ds_error; // 2
}

Possible output:

1: error: static assertion failed: Swap requires copying
2: error: static assertion failed: Data Structure requires 
                                   default-constructible elements

See also

• assert
• type traits

C documentation for static_assert