赋值不同于初始化。

以下两行代码都进行初始化。一个构造函数调用完成:

A a1 = A_factory_func();  // calls copy constructor
A a1(A_factory_func());   // calls copy constructor

但它不等于:

A a1;                     // calls default constructor
a1 = A_factory_func();    // (assignment) calls operator =

我现在还没有一篇文章来证明这一点，但这很容易实验:

#include <iostream>
using namespace std;

class A {
public:
    A() { 
        cout << "default constructor" << endl;
    }

    A(const A& x) { 
        cout << "copy constructor" << endl;
    }

    const A& operator = (const A& x) {
        cout << "operator =" << endl;
        return *this;
    }
};

int main() {
    A a;       // default constructor
    A b(a);    // copy constructor
    A c = a;   // copy constructor
    c = b;     // operator =
    return 0;
}

赋值不同于初始化。

以下两行代码都进行初始化。一个构造函数调用完成:

A a1 = A_factory_func();  // calls copy constructor
A a1(A_factory_func());   // calls copy constructor

但它不等于:

A a1;                     // calls default constructor
a1 = A_factory_func();    // (assignment) calls operator =

我现在还没有一篇文章来证明这一点，但这很容易实验:

#include <iostream>
using namespace std;

class A {
public:
    A() { 
        cout << "default constructor" << endl;
    }

    A(const A& x) { 
        cout << "copy constructor" << endl;
    }

    const A& operator = (const A& x) {
        cout << "operator =" << endl;
        return *this;
    }
};

int main() {
    A a;       // default constructor
    A b(a);    // copy constructor
    A c = a;   // copy constructor
    c = b;     // operator =
    return 0;
}

First grouping: it depends on what A_factory_func returns. The first line is an example of copy initialization, the second line is direct initialization. If A_factory_func returns an A object then they are equivalent, they both call the copy constructor for A, otherwise the first version creates an rvalue of type A from an available conversion operators for the return type of A_factory_func or appropriate A constructors, and then calls the copy constructor to construct a1 from this temporary. The second version attempts to find a suitable constructor that takes whatever A_factory_func returns, or that takes something that the return value can be implicitly converted to.

第二组:逻辑完全相同，只是内建类型没有任何奇异构造函数，因此它们实际上是相同的。

Third grouping: c1 is default initialized, c2 is copy-initialized from a value initialized temporary. Any members of c1 that have pod-type (or members of members, etc., etc.) may not be initialized if the user supplied default constructors (if any) do not explicitly initialize them. For c2, it depends on whether there is a user supplied copy constructor and whether that appropriately initializes those members, but the members of the temporary will all be initialized (zero-initialized if not otherwise explicitly initialized). As litb spotted, c3 is a trap. It's actually a function declaration.

当初始化一个对象时，你可以看到显式和隐式构造函数类型的区别:

类:

class A
{
    A(int) { }      // converting constructor
    A(int, int) { } // converting constructor (C++11)
};

class B
{
    explicit B(int) { }
    explicit B(int, int) { }
};

在main函数中:

int main()
{
    A a1 = 1;      // OK: copy-initialization selects A::A(int)
    A a2(2);       // OK: direct-initialization selects A::A(int)
    A a3 {4, 5};   // OK: direct-list-initialization selects A::A(int, int)
    A a4 = {4, 5}; // OK: copy-list-initialization selects A::A(int, int)
    A a5 = (A)1;   // OK: explicit cast performs static_cast

//  B b1 = 1;      // error: copy-initialization does not consider B::B(int)
    B b2(2);       // OK: direct-initialization selects B::B(int)
    B b3 {4, 5};   // OK: direct-list-initialization selects B::B(int, int)
//  B b4 = {4, 5}; // error: copy-list-initialization does not consider B::B(int,int)
    B b5 = (B)1;   // OK: explicit cast performs static_cast
}

默认情况下，构造函数是隐式的，所以你有两种方法来初始化它:

A a1 = 1;        // this is copy initialization
A a2(2);         // this is direct initialization

通过将结构定义为显式，你只有一种方式是直接的:

B b2(2);        // this is direct initialization
B b5 = (B)1;    // not problem if you either use of assign to initialize and cast it as static_cast

双b1 = 0.5;构造函数的隐式调用。

双b2 (0.5);显式调用。

看看下面的代码，看看区别:

#include <iostream>
class sss { 
public: 
  explicit sss( int ) 
  { 
    std::cout << "int" << std::endl;
  };
  sss( double ) 
  {
    std::cout << "double" << std::endl;
  };
};

int main() 
{ 
  sss ddd( 7 ); // calls int constructor 
  sss xxx = 7;  // calls double constructor 
  return 0;
}

如果类没有显式构造函数，则显式调用和隐式调用是相同的。

值得注意的是:

[12.2/1]类类型的临时对象可以在不同的上下文中创建:…在一些初始化中(8.5)。

例如，用于复制初始化。

[12.8/15]当满足某些条件时，允许实现省略类对象的复制构造…

换句话说，一个好的编译器不会在可以避免的情况下创建副本进行复制初始化;相反，它将直接调用构造函数——即，就像直接初始化一样。

换句话说，复制初始化就像在大多数<opinion>的情况下直接初始化，其中已经编写了可理解的代码。由于直接初始化可能会导致任意(因此可能是未知的)转换，我更喜欢在可能的情况下总是使用复制初始化。(额外的是，它实际上看起来像初始化。)</opinion>

技术血淋淋的景象: [12.2/1接续以上]即使避免了临时对象的创建(12.8)，也必须遵守所有的语义限制，就像创建了临时对象一样。

很高兴我不是在写c++编译器。