C++中不同的声明方式有什么区别？

Question

What is the difference between the following declarations?

C c1 {C(42)};       
C c2 (C(42));    
C c3 {7};   
C c4 = {7};   
C c5 (7);

where C is:

struct C {
    int x;
    C(int n): x{n} {};
    C(const C &c) {x=c.x;};
};   

Which constructor will be called or whether some temporary object will be created?

Answer 1

Let's make a simple change to your struct so that it prints which constructor is being called:

struct C {
    int x;
    C(int n): x{n} { cout << "calling C(int n) \n"; };
    C(const C &c) { x=c.x; cout << "calling C(const C &c) \n"; };
};   

C c1 {C(42)};
C c2 (C(42)); 
C c3 {7};   
C c4 = {7};
C c5 (7);  

// output:
// calling C(int n)
// calling C(int n)
// calling C(int n)
// calling C(int n)
// calling C(int n)

C c6(c1); // this one uses the copy constructor

// output:
// calling C(const C &c)

Answer 2

C c1 {C(42)};
// not sure this works

C c2 (C(42));
// calls the constructor of C with 42 and then copies the object that temporary created to the c2 
// object b calling the copy constructor of C

C c3 {7};
// not sure this works

C c4 = {7};
// not sure this works
// it should be and array like :
// C c4[] = {C(7)} ;
// to create and array of objects with initialized values ( C ( 7 ) )

C c5 (7);
// will create c5 with initialized value x = 5