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User-defined literals (since C++11)

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C++11 introduced the ability to add custom suffixes to literals in order to provide different values. Literal suffixes can be overloaded in a way very similar to operators.

Syntax

return decl operator"" name ( unsigned long long n ) { body }
return decl operator"" name ( long double d ) { body }
return decl operator"" name ( char c ) { body }
return decl operator"" name ( const char* str, size_t sz ) { body }
return decl operator"" name ( const char* cstr ) { body }

Explanation

return - Return value
decl - Declaration specifier sequence, can contain keywords as constexpr or inline
name - A valid C++ identifier, prefixed with an underscore. Identifiers without underscore are reserved for future use
n - Value resulting from an integral literal
d - Value resulting from a floating-point literal
c - Value resulting from a character literal
cstr - Null-terminated string as parsed by the compiler, for integer and floating point literals
str/sz - Buffer and size from a string literal
body - Function body

Examples

// used as conversion
inline constexpr long double operator"" _deg ( long double deg )
{
    return deg*3.141592/180;
}
...
double x = 90.0_deg; // x = 1.570796


// used with custom type
struct mytype
{
    ...
    mytype ( unsigned long long );
};
mytype operator"" _mytype ( unsigned long long n )
{
    return mytype(n);
}
...
mytype x = 123_mytype;


// used for side-effects
void operator"" _print ( const char* str )
{
    std::cout << str;
}
...
0x123ABC_print;

Output:

0x123ABC