Explicit constructor taking multiple arguments

c++ explicit-constructor
Peter G. picture Peter G. · Aug 24, 2016 · Viewed 12.2k times · Source

Does making a constructor having multiple arguments explicit have any (useful) effect?

Example:

class A {
    public:
        explicit A( int b, int c ); // does explicit have any (useful) effect?
};

Answer

Sneftel picture Sneftel · Aug 24, 2016

Up until C++11, yeah, no reason to use explicit on a multi-arg constructor.

That changes in C++11, because of initializer lists. Basically, copy-initialization (but not direct initialization) with an initializer list requires that the constructor not be marked explicit.

Example:

struct Foo { Foo(int, int); };
struct Bar { explicit Bar(int, int); };

Foo f1(1, 1); // ok
Foo f2 {1, 1}; // ok
Foo f3 = {1, 1}; // ok

Bar b1(1, 1); // ok
Bar b2 {1, 1}; // ok
Bar b3 = {1, 1}; // NOT OKAY

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