c++ - Why I am not getting error in following code? -

#include <iostream>  using namespace std;  class { private:     float data_member; public:     a(int a);     explicit a(float d); };   a::a(int a) {     data_member = a; }  a::a(float d) {     data_member = d; }  void test(a a) {     cout<<"do nothing"<<endl; }  int main() {     test(12);     test(12.6); //expecting compile time error here     return 0; } 

i expecting error int case ctor takes float value explicit. not getting error in vs 2010. please point me out if wrong understanding of keyword "explicit" in c++.

explicit a(float d); 

does not think does. disables implicit conversion float type a. in short, disables implicit conversion wherein float implicitly converted object of a. ex:

void dosomething(a obj){}  dosomething(2.3); 

it not disable implicit conversions allowed standard.

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why compile?

test(12.6); 

because float parameter implicitly converted int. happens behind scenes same as:

float = 12.6; int   b = (int)a; 

further, conversion constructor a::a(int a) used create object of type a passed method test().

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why not compile if remove explicit?

without keyword explicit conversion constructor a::a(float d) available conversions , creates ambiguity because there 2 possible matches, when converting 12.6 object of type a:

a::a(int a) 

or

a::a(float d) 

since none scores on other in terms of best match, compiler emits diagnostic of ambiguity.