類的成員變數 宣告順序 決定 初始化順序(建構函式初始化列表不影響)
阿新 • • 發佈:2019-01-29
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