Direct Initialization vs. Value Initialization

Question

I am a C programmer trying to learn C++11, and I've run into something I don't understand. From what I can tell, the following issue is a difference between value initialization and direct initialization.

The following code snippet does not compile using Visual Studio:

class TestClass {
    int _val;
    std::string _msg;
public:
    TestClass(int, std::string);
    void action();
};

TestClass::TestClass(int val, std::string msg)
    : _val{val}, _msg{msg}
{
}

void TestClass::action()
{
    std::cout << _msg << _val << std::endl;
}

It gives me:

error C2797: 'TestClass::_msg': list initialization inside member initializer list or non-static data member initializer is not implemented

However, changing

TestClass::TestClass(int val, std::string msg)
    : _val{val}, _msg{msg}

to

TestClass::TestClass(int val, std::string msg)
    : _val{val}, _msg(msg)

fixes my problem. What is the difference between these two forms of initialization, and when should one be used over the other? I've been led to believe that I should use value initialization whenever dealing with explicit types.

There's nothing wrong with your first example. Your compiler is wrong. What version are your using? **Edit** Scratch that. The error message says "not implemented". So the compiler authors are aware of the problem, they just haven't fixed it yet in the version you are using. — Benjamin Lindley, Jul 12 '15 at 00:04
http://en.cppreference.com/w/cpp/language/list_initialization — David G, Jul 12 '15 at 00:08
both `_msg{msg}` and `_msg(msg)` are [direct initialization](http://en.cppreference.com/w/cpp/language/direct_initialization). The difference is that the former is [direct-list-initialization](http://en.cppreference.com/w/cpp/language/list_initialization), while the latter is, well, direct-non-list-initialization. — cpplearner, Jul 12 '15 at 00:12
Strangely, I've seen similar differences between gcc-4.9 on mingw-w64 versus the same version on Linux. WTF. For me it was initialization of std::array members. — emsr, Jul 12 '15 at 02:29

score 3 · Accepted Answer · answered Jul 12 '15 at 00:09

This is an implementation detail of the Visual C++ compiler. You can read more about this error here. The page states:

The C++ compiler in Visual Studio does not implement list initialization inside either a member initializer list or a non-static data member initializer

Your code attempts to implement the first case. The solution you proposed solves this problem, but if you still prefer to use the initializer list somehow in the constructor, you can do this:

TestClass::TestClass(int val, std::string msg)
    : _val{val}, _msg(std::string{msg})

Which will work as you intend.

Direct Initialization vs. Value Initialization

1 Answers1