为什么在以下代码中没有出现错误？

Question

#include <iostream>

using namespace std;

class A
{
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 a compile time error here
    return 0;
}

I am expecting a error int this case as my CTOR that takes float value is explicit. But I am not getting any error in VS 2010. Please point me out if I am wrong with my understanding of keyword "EXPLICIT" in c++.

Answer 1

explicit A(float d);

Does not do you think it does. It disables the implicit conversion from float to the type A . In short, It disables any implicit conversion wherein a float will be implicitly converted to a object of A . For ex:

void doSomething(A obj){}

doSomething(2.3);

It does not disable any implicit conversions allowed by the standard.

---

Why does it compile?

Test(12.6);

Because the float parameter is implicitly converted to int . What happens behind the scenes is same as:

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

Further, the conversion constructor A::A(int a) is used to create a object of type A which is passed to the method Test() .

---

Why does it not compile if you remove the explicit ?

Without the keyword explicit the conversion constructor A::A(float d) is available for conversions and this creates a ambiguity because there are two possible matches, when converting 12.6 to object of type A :

A::A(int a)

or

A::A(float d)

Since none scores over other in terms of best match, the compiler emits the diagnostic of ambiguity.