expected ‘,’ or ‘...’ before string constant

Why const std::string str="__$HOOK_FUNC_FOR_LUA_KEY@__"; is ok whereas the compiler complains when compiling

const std::string str("__$HOOK_FUNC_FOR_LUA_KEY@__");?

NOTE: str is a member variable of a C++ class.

Demo code:

class Demo
{
private:
const std::string str("__$HOOK_FUNC_FOR_LUA_KEY@__");
}

Here are the the error messages:

test.hpp:253:51: error: expected identifier before string constant
     const std::string str("__$HOOK_FUNC_FOR_LUA_KEY@__");
                                                   ^
test.hpp:253:51: error: expected ‘,’ or ‘...’ before string constant

It's obvious that there is ctor for std::string(char*). So I am really confused.