c++新特性之explicit

explicit for ctors taking one argument

我们看一组例子 有两个复数类 Complex 与Complex2 他们的唯一区别就是

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struct Complex
{
    int real,image;
    Complex(int a, int b=0) : real(a), image(b)
    {
    }
    Complex operator+(const Complex &x)
    {
        return Complex(real + x.real, image + x.image);
    }
};

struct Complex2
{
    int real,image;
    explicit Complex2(int a, int b=0) : real(a), image(b)
    {
    }
    Complex2 operator+(const Complex2 &x)
    {
        return Complex(real + x.real, image + x.image);
    }
};
``` 

```cpp
int main()
{
    Complex c1(12,4);
    Complex c2 = c1 + 5; //ok 5被转隐式转换为Complex
    
    Complex2 c3(12,4);
    Complex2 c4 = c3 +5; //error 
    return 0;
}

explicit for ctors taking more than one argument (c++11)