what's the difference between the two copy constructor invocation in C++?

Question

Look at the following codes:

class Foo    
{ 
public:  
    Foo(){}  
    explicit Foo(const Foo &){}  
};  
int main()  
{  
    Foo foo1;  
    Foo foo2(foo1);  
    Foo foo3 = foo1; //can not compile    
    return 0;  
}  

Why Foo foo3 = foo1; can not compile, and what's the difference between the two copy constructor invocation?
ps: My compiler tools is GCC4.8.2

Answer 1

Foo foo2(foo1); is direct initialization. Foo foo3 = foo1; is copy initialization. The difference between them is

Copy-initialization is less permissive than direct-initialization: explicit constructors are not converting constructors and are not considered for copy-initialization.

The copy constructor of Foo is declared as explicit, which is not considered in copy initialization.