c++ - Class instance inside class declaration with constant arguments

Question

I am having trouble declaring instances of a class with constant arguments inside the definition of another class.

class Foo
{
    private:
        const int m_a, m_b;
    public:
        Foo(int a, int b) : m_a(a), m_b(b) {}
};

class Bar
{
    public:
        Foo foo1(1,2);
        Foo foo2(2,3);
};

From this I get the errors:

"error: expected identifier before numeric constant"
"error: expected ',' or '...' before numeric constant"

Answer 1

这不是您初始化数据成员的方式。您必须在构造函数的初始化列表中执行此操作：

class Bar
{
public:
    Bar() : foo1(1,2), foo2(2, 3) { }
    //    ^^^^^^^^^^^^^^^^^^^^^^^
    //    This is how you initialize the sub-objects of a class
    //    (both base sub-objects and member sub-objects)
    Foo foo1;
    Foo foo2;
};

请注意，C++11 确实允许对数据成员进行内联初始化，但语法略有不同（您需要使用大括号）：

class Bar
{
public:
    Foo foo1{1, 2};
    Foo foo2{2, 3};
};

或者，您也可以使用复制初始化语法：

class Bar
{
public:
    Foo foo1 = Foo(1, 2);
    Foo foo2 = Foo(2, 3);
};

Answer 2

您必须通过构造函数初始化这些成员：

class Bar
{
    public:
        Foo foo1;
        Foo foo2;

        Bar() : foo1(1, 2), foo2(2, 3) {}
};

冒号后面是成员初始化器列表。

Answer 3

在 C++03 中，Bar需要在其构造函数中初始化其属性：

class Bar
{
    public:
        Foo foo1;
        Foo foo2;

        Bar() : foo1(1,2), foo2(2,3) {} 
};