類成員的宣告順序決定初始化順序；
建構函式初始化列表不影響初始化順序；

C++初始化類成員時，是按照宣告的順序初始化的，而不是按照出現在初始化列表中的順序。

class B
{
public:
    //m_b = 2,m_a = 1
    B():m_a(1),m_b(m_a+1)
    {
        cout << "m_b = " << m_b << ",m_a = " << m_a << endl;
    }
    //m_b = 2,m_a = 1
    B():m_b(m_a+1),m_a(1)
    {
        cout
 
 << "m_b = " << m_b << ",m_a = " << m_a << endl;
    }
    //上述兩個結果一樣的，可以判斷初始化順序由宣告順序決定。

    //m_b = 1,m_a = -1218015231
    B():m_b(1),m_a(m_b+1)
    {
        cout << "m_b = " << m_b << ",m_a = " << m_a << endl;
    }
private:
    //宣告順序
    int
 
 m_a;
    int m_b;
};
int main()
{
    B b;
}

例子：

#include <iostream>
using namespace std;

class A
{
public:
    A():a(1),b(8){cout<< "A()"<<endl;}
    int a;
    int b;
};

class B
{
public:
    B():m_b(a.b),m_a(a.a) //建構函式初始化列表
    {
    cout << "m_b = " << m_b << ",m_a = "
 
 << m_a << endl;
    }

private:
    //下列為：宣告順序
    int m_a;
    //A a;
    int m_b;
    A a;
};

class C: public B
{};

int main()
{
    B b;
    C c;
    return 1;
}

不同情況，兩種結果：
當使用//A a;時，下面結果：m_a為隨機值

A()
m_b = 8,m_a = 0
A()
m_b = 8,m_a = -1076785724

當使用A a;時，下面結果：m_a、m_b為隨機值

A()
m_b = -1218158592,m_a = 134514971
A()
m_b = -1219712267,m_a = -1076671652