What's the difference between explicit and implicit assignment in C++

问题

int value = 5; // this type of assignment is called an explicit assignment
int value(5); // this type of assignment is called an implicit assignment

What is the difference between those, if any, and in what cases do explicit and implicit assignment differ and how?

http://weblogs.asp.net/kennykerr/archive/2004/08/31/Explicit-Constructors.aspx

EDIT: I actually just found this article, which makes the whole thing a lot clearer... and it brings up another question, should you (in general) mark constructors taking a single parameter of a primitive type - numeric/bool/string - as explicit and leave the rest as they are (of course keeping watch for gotchas such as constructors like (int, SomeType = SomeType())?

回答1:

They differ if a class has a constructor marked 'explicit'. Then, one of these does not work.

Otherwise, no difference.

回答2:

Neither of these is an assignment of any kind -- they're both initialization. The first uses copy initialization, and the second direct initialization. (FWIW, I'm pretty sure I've never heard the terms "explicit assignment" or "implicit assignment" before).

Edit: (Mostly in response to Nathan's comment):

Here's a corrected version of the code from your comment:

#include <iostream>

struct Foo { 
    Foo() { 
        std::cout << "Foo::ctor()" << std::endl; 
    } 
    Foo(Foo const& copy) { 
        std::cout << "Foo::cctor()" << std::endl; 
    } 
    Foo& operator=(Foo const& copy) { 
        std::cout << "foo::assign()" << std::endl; 
        return *this; 
    } 
};

int main(int, const char**) { 
    Foo f; 
    Foo b(f); 
    Foo x = b;
    return 0; 
}

The result from running this should be:

Foo::ctor()
Foo::cctor()
Foo::cctor()

If you run it and get an foo::assign(), throw your compiler away and get one that works (oh, and let us know what compiler it is that's so badly broken)!

回答3:

Only the first one is an assignment. They are both initialization.

Edit: actually, I'm wrong. Neither are assignment.

来源：https://stackoverflow.com/questions/3057859/whats-the-difference-between-explicit-and-implicit-assignment-in-c

标签

c++

explicit-constructor