Different ways of initializing an object in c++

问题

I'm pretty sure this is a duplicate question, but I've been searching for a while and I didn't get any smarter.

Imagine this class:

class Entity {
public:
    int x, y;

    Entity() : x(0), y(0) { }
    Entity(int x, int y) : x(x), y(y) { }
}

And here are multiple ways of initializing the class:

Entity ent1;
Entity ent2();
Entity ent3(1, 2);
Entity ent4 = Entity();
Entity ent5 = Entity(2, 3);

I also know that's it's possible make an object on the heap memory, but that's not a great mystery to me at this moment.

Here's what I think I know

ent1 - Uses the default constructor, so x=0 and y=0

ent2 - Uses the default constructor, so x=0 and y=0 (Not sure)

ent3 - Made constructor, so x=1 and y=2

ent4 - Default constructor, so x=0 and y=0

ent5 - Made constructor, so x=2 and y=3

Correct me if I'm wrong. But my question is, what's the difference between these ways of initializing an object?

I'm not sure which one I should use when.

回答1:

Entity ent1;

This statement uses default constructor of class Entity.

Entity ent2();

This definition will be treated by compiler as a function prototype if that's possible.

Entity ent3(1, 2);

User-defined constructor is invoked for ent3.

Entity ent4 = Entity();

This is a proper version of ent2 case. Default constructor is invoked as part of value initialization. Its form allows to avoid Most Vexing Parse - ambiguity solving principle which makes ent2 incorrect.

Entity ent5 = Entity(2, 3);

A version of ent3 case. User-defined constructor invoked as part of value initialization.

Your question is tagged as C++11, and C++11 allows uniform initialization syntax:

Entity ent12{};     // This is a legal alternative of ent2 case
Entity ent13{1, 2};
Entity ent14 = {};
Entity ent15 = Entity{2, 3};

Note that uniform initialization syntax has a caveat. E.g. this line

std::vector<int> v(10);

declares a vector of 10 elements. But this one

std::vector<int> v{10};

declares a vector initialized with single element of type int with value 10. This happens because std::vector has a constructor with following signature defined:

vector( std::initializer_list<T> init, const Allocator& alloc = Allocator() );

In case that you can't use neither () without triggering MVP nor {} without invoking undesired constructor, the value initialization assignment syntax allows to resolve the issue.

来源：https://stackoverflow.com/questions/49802012/different-ways-of-initializing-an-object-in-c

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c++

class

c++11

constructor

initialization