class Foo {
public:
Foo(float b) {}
};
class Bar {
public:
Bar(Foo foo) {}
};
int main(int argc,char *argv[]) {
Bar b1(3.0f); // accept,one implicit convertion happens there.
Bar b2 = 3.0f; // error: no viable conversion from 'float' to 'Bar'
return 0;
}
解决方法
来自[dcl.init]：
Otherwise (i.e.,for the remaining copy-initialization cases),user-defined conversion sequences
that can convert from the source type to the destination type or (when a conversion function
is used) to a derived class thereof are enumerated as described in 13.3.1.4,and the best one is
chosen through overload resolution (13.3).
我们可以调用从源类型直接到目标类型的用户定义转换.也就是说,如果我们有Bar(float),我们会考虑构造函数.但是,在这种情况下,我们的候选人只是Bar(Foo),它没有浮动.
您可以进行零或一个用户定义的转换.在直接初始化的情况下,我们只需调用Bar(Foo),它调用一个用户定义的转换(float – > Foo).在复制初始化的情况下,我们正在寻找从float(源类型)一直到Bar(目标类型)的转换序列,这将涉及两个用户定义的转换(float – > Foo,Foo – – > Bar),因此错误.
