initialization: parenthesis vs. equals sign [duplicate]

by 
T a( b );

is direct initialization, unless it parses as a function declaration, in which case it’s a function declaration.

T a = b;

is copy initialization, which means that it works as if a temporary object is constructed on the right hand side, and that a is then copy constructed or, in C++11 and later, possibly move constructed, from that temporary.

The compiler is free to elide (remove) the temporary+copying/moving whenever it can, but a copy or move constructor, whichever would be logically used, must still be accessible and not explicit.

For example, in C++03 you cannot copy-initialize a std::ostringstream, because it doesn’t have a copy constructor. In C++11 you can copy-initialize an ostringstream if the initializer is a temporary, which then results in a logical move construction (which however will usually be elided, optimized away). For example, this copy initialization declaration,

ostringstream s = ostringstream( "blah" );

… doesn’t compile as C++03, because in C++03 the copy initialization invokes the class’ copy constructor, which doesn’t exist. It does however compile as C++11, because in C++11 the copy initialization invokes the move constructor. And while (to maintain its illusion of being a stream) a std::ostringstream can’t be directly copied, it can be moved.

Another such difference: in C++03 only the copy initialization syntax supports curly braces initializer, which in C++03 you can use when T is an aggregate type such as a raw array. In C++11 the curly braces notation has been extended and generalized as a uniform initialization syntax, so it can be used also with direct initialization. And so the following direct initialization declaration,

int v[]{ 3, 1, 4, 1, 5, 9, 2, 6, 5, 4 };

… does not compile as C++03, but does compile as C++11 and later.

The = copy initialization syntax is the original initialization syntax from C.

And in C++11 and later, due to move semantics, it can be used in a much wider range of cases than in C++03, such as with a std::ostringstream.

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