C++：没有匹配的调用函数：为什么需要一个空的构造函数？

Question

当我尝试编译以下代码时：

class a {

  int i;

  public :
  a(int);
};

class b {
  a mya;  
  int j;

  public:
  b(int);

};

a::a(int i2) {
  i=i2;
}

b::b(int i2) {
  mya=a(i2); 
  j=2*i2;
}

int main() {

}

我收到以下错误：

prog.cpp:21:12: error: no matching function for call to ‘a::a()

 b::b(int i2) {
            ^
prog.cpp:17:1: note: candidate: ‘a::a(int)

 a::a(int i2) {
 ^
prog.cpp:17:1: note:   candidate expects 1 argument, 0 provided
prog.cpp:1:7: note: candidate: ‘constexpr a::a(const a&)’
 class a {
       ^
prog.cpp:1:7: note:   candidate expects 1 argument, 0 provided
prog.cpp:1:7: note: candidate: ‘constexpr a::a(a&&)

prog.cpp:1:7: note:   candidate expects 1 argument, 0 provided

似乎期望 class a 没有参数的构造函数。我不明白为什么，我唯一一次创建类型为 a 的对象时，我调用了以 int 作为参数的构造函数。

我知道解决方案是为 a 添加一个不带参数的构造函数。但是为什么？

感谢您的回答，最好的问候，

杰罗姆

Answer 1

在b的构造函数中，mya=a(i2);是赋值（不是初始化）。在进入构造函数体之前，mya被尝试默认初始化但是a没有默认构造函数。

正如您所说，您可以为 a 添加默认构造函数，然后 mya 将被默认初始化，然后在 b.

的构造函数中进行赋值

更好的方法是在member initializer list中初始化mya。

b::b(int i2) : mya(i2) {
//           ^^^^^^^^^
  j=2*i2; // this could be moved to member initializer list too
}

Answer 2

_{（以下所有 ISO 标准参考均指 N4659: March 2017 post-Kona working draft/C++17 DIS）}

根据 [class.base.init]/9，b 的 mya 成员是 a 类型，默认初始化，但 a 没有定义默认构造函数：

In a non-delegating constructor, if a given potentially constructed subobject is not designated by a mem-initializer-id (including the case where there is no mem-initializer-list because the constructor has no ctor-initializer), then

(9.1) if the entity is a non-static data member that has a default member initializer [...] the entity is initialized from its default member initializer as specified in [dcl.init];

(9.2) otherwise, if the entity is an anonymous union or a variant member ([class.union.anon]), no initialization is performed;

(9.3) otherwise, the entity is default-initialized.

此处，由于 mya 未与默认成员初始值设定项一起声明，因此适用 [class.base.init]/9.3。

[class.base.init]/9 的示例甚至涵盖了这种特殊情况：

[...] [ Example:

struct A {
  A();
};

struct B {
  B(int);
};

struct C {
  C() { }               // initializes members as follows:
  A a;                  // OK: calls A::A()
  const B b;            // error: B has no default constructor
  int i;                // OK: i has indeterminate value
  int j = 5;            // OK: j has the value 5
};

— end example ]

您可以通过为 mya 提供默认成员初始值设定项来解决它，这样 [class.base.init]/9.1 就适用

class b {
  a mya{42};  // default member initializer
  int j;

  public:
  b(int);
};

或者，在b的构造函数定义中使用成员初始化列表； b::b(int)，这样 [class.base.init]/7 适用：

The expression-list or braced-init-list in a mem-initializer is used to initialize the designated subobject (or, in the case of a delegating constructor, the complete class object) according to the initialization rules of [dcl.init] for direct-initialization. [ Example:
struct B1 { B1(int); /* ... */ };
struct B2 { B2(int); /* ... */ };
struct D : B1, B2 {
  D(int);
  B1 b;
  const int c;
};

D::D(int a) : B2(a+1), B1(a+2), c(a+3), b(a+4) { /* ... */ }
D d(10);
— end example ] [...]

因此直接初始化 mya 成员：

b::b(int i2) : mya(i2) {
           //  ^^^^^^^- member initializer list
  j=2*i2;
}

C++：没有匹配的调用函数：为什么需要一个空的构造函数？

C++: no matching function for call: why is an empty constructor needed?

c++

constructor

default-constructor

c++11