Namespaces
Variants
Views
Actions

Static Assertion

From cppreference.com
< cpp‎ | language
Revision as of 08:00, 4 April 2014 by D41D8CD98F (Talk | contribs)

(diff) ← Older revision | Latest revision (diff) | Newer revision → (diff)

Performs compile-time assertion checking

Contents

[edit] Syntax

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

[edit] 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)

[edit] 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.

[edit] Example

#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

[edit] See also