【C++】38逻辑操作符的陷阱 不要重载逻辑操作符

潜规则

操作符只有两种值（true和false）

逻辑表达式不能完全计算就能确定最终值

最终结果只能是true或者false

例：逻辑表达式：

#include <iostream>
#include <string>

using namespace std;

int func(int i)
{
    cout << "int func(int i) : i = " << i << endl;
    
    return i;
}

int main()
{
    if( func(0) && func(1) )
    {
        cout << "Result is true!" << endl;
    }
    else
    {
        cout << "Result is false!" << endl;
    }
    
    cout << endl;
    
    if( func(0) || func(1) )
    {
        cout << "Result is true!" << endl;
    }
    else
    {
        cout << "Result is false!" << endl;
    }
    
    return 0;
}

输出：

int func(int i) : i = 0
Result is false!

int func(int i) : i = 0
int func(int i) : i = 1
Result is true!

重载逻辑操作符

C++通过函数调用扩展操作符的功能

进入函数体前必须完成所有参数的计算

函数参数的计算次序是不定的

短路法则完全失效

#include <iostream>
#include <string>

using namespace std;

class Test
{
    int mValue;
public:
    Test(int v)
    {
        mValue = v;
    }
    int value() const
    {
        return mValue;
    }
};

bool operator && (const Test& l, const Test& r)
{
    return l.value() && r.value();
}

bool operator || (const Test& l, const Test& r)
{
    return l.value() || r.value();
}

Test func(Test i)
{
    cout << "Test func(Test i) : i.value() = " << i.value() << endl;
    
    return i;
}

int main()
{
    Test t0(0);
    Test t1(1);
    
    if( func(t0) && func(t1) )
    {
        cout << "Result is true!" << endl;
    }
    else
    {
        cout << "Result is false!" << endl;
    }
    
    cout << endl;
    
    if( func(1) || func(0) )
    {
        cout << "Result is true!" << endl;
    }
    else
    {
        cout << "Result is false!" << endl;
    }
    
    return 0;
}

输出结果：

Test func(Test i) : i.value() = 1
Test func(Test i) : i.value() = 0
Result is false!

Test func(Test i) : i.value() = 0
Test func(Test i) : i.value() = 1
Result is true!

例：

#include <iostream>
#include <string>

using namespace std;

class Test
{
    int mValue;
public:
    Test(int v)
    {
        mValue = v;
    }
    int value() const
    {
        return mValue;
    }
};

bool operator && (const Test& l, const Test& r)
{
    return l.value() && r.value();
}

bool operator || (const Test& l, const Test& r)
{
    return l.value() || r.value();
}

Test func(Test i)
{
    cout << "Test func(Test i) : i.value() = " << i.value() << endl;
    
    return i;
}

int main()
{
    Test t0(0);
    Test t1(1);
    
    if( operator && (func(t0),func(t1) ))
    {
        cout << "Result is true!" << endl;
    }
    else
    {
        cout << "Result is false!" << endl;
    }
    
    cout << endl;
    
      if( operator || (func(1),func(0) ))
    {
        cout << "Result is true!" << endl;
    }
    else
    {
        cout << "Result is false!" << endl;
    }
    
    return 0;
}

输出：

Test func(Test i) : i.value() = 1
Test func(Test i) : i.value() = 0
Result is false!

Test func(Test i) : i.value() = 1
Test func(Test i) : i.value() = 0
Result is true!

有用的建议：

实际工程开发中避免重载逻辑操作符

通过重载比较操作符代替逻辑操作符重载

直接使用成员函数代替逻辑操作符的重载

使用全局函数对逻辑操作符进行重载