Difference between revisions of "cpp/language/explicit"
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Specifies constructors and {{mark since c++11}} conversion operators that don't allow [[cpp/language/implicit_cast|implicit conversions]] or [[cpp/language/copy_initialization|copy-initialization]]. | Specifies constructors and {{mark since c++11}} conversion operators that don't allow [[cpp/language/implicit_cast|implicit conversions]] or [[cpp/language/copy_initialization|copy-initialization]]. | ||
Revision as of 07:37, 16 December 2014
Specifies constructors and (since C++11) conversion operators that don't allow implicit conversions or copy-initialization.
Syntax
| explicit class_name ( params ) | |||||||||
| explicit operator type ( ) (since C++11) | |||||||||
1) specifies that this constructor is only considered for direct initialization (including explicit conversions)
2) specifies that this user-defined conversion function is only considered for direct initialization (including explicit conversions)
Notes
A constructor with a single non-default parameter(until C++11) that is declared without the function specifier explicit is called a converting constructor.
An explicit default constructor can be used to perform both default initialization and value initialization: the use of explicit with a default constructor has no effect unless it has default arguments that would allow it to be used in copy-initialization context.
Example
Run this code
struct A { A(int) {} // converting constructor A(int, int) {} // converting constructor (C++11) operator int() const { return 0; } }; struct B { explicit B(int) {} explicit B(int, int) {} explicit operator int() const { return 0; } }; int main() { A a1 = 1; // OK: copy-initialization selects A::A(int) A a2(2); // OK: direct-initialization selects A::A(int) A a3 {4,5}; // OK: direct-list-initialization selects A::A(int, int) A a4 = {4,5}; // OK: copy-list-initialization selects A::A(int, int) int na1 = a1; // OK: copy-initialization selects A::operator int() int na2 = static_cast<int>(a1); // OK: static_cast performs direct-initialization A a5 = (A)1; // OK: explicit cast performs static_cast // B b1 = 1; // Error: copy-initialization does not consider B::B(int) B b2(2); // OK: direct-initialization selects B::B(int) B b3 {4,5}; // OK: direct-list-initialization selects A::A(int, int) // B b4 = {4,5}; // Error: copy-list-initialization does not consider B::B(int,int) // int nb1 = b2; // Error: copy-initialization does not consider B::operator int() int nb2 = static_cast<int>(b2); // OK: static_cast performs direct-initialization B b5 = (B)1; // OK: explicit cast performs static_cast }
