c++ - Initialization by user-defined constructor -

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consider following program:

#include <iostream>  struct {     a(int, int){ std::cout << "a(int, int)" << std::endl; } };  a(4,4); b{4,4}; c({4,4});  int main(){ }

output:

a(int, int) a(int, int) a(int, int)

demo

i' m intereste in there difference between initialization of a, b , c? of theme direct initialized.

a a(4,4);

bog standard direct-initialization.

a b{4,4};

bog standard direct-list-initialization. since a has no initializer_list constructor, ends doing same thing above. no std::initializer_list object ever constructed or destroyed.

a c({4,4});

this direct-initializes c a temporary, in turn copy-list-initialized braced-init-list {4, 4}. note won't work if a::a(int, int) explicit, since in case copy-list-initialization ill-formed , left no viable constructor call. construction , destruction of temporary may elided, , is.


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