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static_assert

Tests a software assertion at compile time. If the specified constant expression is false, the compiler displays the specified message and the compilation fails with an error; otherwise, the declaration has no effect.

static_assert( 
    constant-expression, 
    string-literal 
);

Parameter

Description

constant-expression

An integral constant expression that can be converted to a Boolean.

If the evaluated expression is zero (false), the string-literal parameter is displayed and the compilation fails with an error. If the expression is nonzero (true), the static_assert declaration has no effect.

string-literal

An message that is displayed if the constant-expression parameter is zero. The message is a string of characters in the base character set of the compiler; that is, not multibyte or wide characters.

The constant-expression parameter of a static_assert declaration represents a software assertion. A software assertion specifies a condition that you expect to be true at a particular point in your program. If the condition is true, the static_assert declaration has no effect. If the condition is false, the assertion fails, the compiler displays the message in string-literal parameter, and the compilation fails with an error.

The static_assert declaration tests a software assertion at compile time. In contrast, the assert (CRT) macro tests a software assertion at run time and incurs a run time cost in space or time. The static_assert declaration is especially useful for debugging templates because template arguments can be included in the constant-expression parameter.

The compiler examines the static_assert declaration for syntax errors when the declaration is encountered. The compiler evaluates the constant-expression parameter immediately if it does not depend on a template parameter. Otherwise, the compiler evaluates the constant-expression parameter when the template is instantiated. Consequently, the compiler might issue a diagnostic message once when the declaration is encountered, and again when the template is instantiated.

You can use the static_assert keyword at namespace, class, or block scope. (The static_assert keyword is technically a declaration, even though it does not introduce new name into your program, because it can be used at namespace scope.)

Description

In the following example, the static_assert declaration has namespace scope. Because the compiler knows the size of type void *, the expression is evaluated immediately.

Example

static_assert(sizeof(void *) == 4, "64-bit code generation is not supported.");

Description

In the following example, the static_assert declaration has class scope. The static_assert verifies that a template parameter is a plain old data (POD) type. The compiler examines the static_assert declaration when it is declared, but does not evaluate the constant-expression parameter until the basic_string class template is instantiated in main().

Example

#include <type_traits>
#include <iosfwd>
namespace std {
template <class CharT, class Traits = std::char_traits<CharT> >
class basic_string {
    static_assert(tr1::is_pod<CharT>::value,
                  "Template argument CharT must be a POD type in class template basic_string");
    // ...
    };
}
struct NonPOD {
    NonPOD(const NonPOD &) {}
    virtual ~NonPOD() {}
};
int main()
{
    std::basic_string<char> bs;
}

Description

In the following example, the static_assert declaration has block scope. The static_assert verifies that the size of the VMPage structure is equal to the virtual memory pagesize of the system.

Example

#include <sys/param.h> // defines PAGESIZE
class VMMClient {
public:
    struct VMPage { // ... 
           };
    int check_pagesize() {
    static_assert(sizeof(VMPage) == PAGESIZE,
        "Struct VMPage must be the same size as a system virtual memory page.");
    // ...
    }
// ...
};

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