Pitfalls and Fallacies¶

约 423 个字 72 行代码预计阅读时间 2 分钟

Anonymous namespace 作用域为当前文件（相当于全局静态变量）

#include<iostream>

using namespace std;

int x = 5;

namespace{
    int x = 10;
}

int main(){
    cout << "x = " << ::x << endl;
    return 0;
}

// ::x = 10

Equivocal function overload

值传递与引用传递

void f(int x){}
void f(int &x)

默认参数

double f(double x, double y = 1.0){return 1;}
double f(double x){return 2;}

隐式转换

int f(unsigned int x) {return x;}
double f(double x) {return x;}
int a = 5;
f(5);

上述所有情况，不会编译错误。只有产生调用时，才会编译错误。

const

下面两种都是 typename的常量类型

const <typename> <var name>;
<typename> const <var name>;

指针与常量同时出现，遵循左结合规则

const int * -> (const int) *
int * const -> (int *) const

new

with initialization
```
p=new T(init list);
```
不能在类中对数据成员初始化
freind

访问权限对友元函数无效，友元函数放在private, public, protected都行

const member and static member

普通常量只能在初始化列表初始化

class A
{
 const int a;
 static const int b;
};

const int A::b = 5;

const member function

注意const是参数表的一部分，const A* this
```
void A::f() const {}
```
Derived class

派生类中的同名函数，不会导致父类的东西消失。仍可通过Base::f 访问

Virtual inheritance

Diamond inheritance construct：

class A{};
class B_1: virtual public A{}
class B_2: virtual public A{}
class C: public B_1, public B_2{}

A
B_1
B_2
C

Derived class and Base class

Derived d;
// value
Base b = d; // truncate
// 下列仅在public继承时可以
// reference
Base &br = d;
// pointer
Base *pb = &d;

++ overload

a++;
operator ++(int){}

++a;
operator ++(){}

重载类型转换时，不需要写返回值类型
静态成员函数，不可以为虚函数，构造函数不可以时虚函数
Virtual function in derived class must have the same signature as in the base class

If only differ from return values, Compile error.

If differ from arg list, normal name hiding.
当类中至少有一个纯虚函数，那么这个类就是抽象类
dynamic cast, cast from base pointer/reference to derived pointer/reference

Pointer failed: return null pointer.

Reference failed: exception.
Only fully specialization is allowed in function templates.
Operators can not be overloaded:

? . :: sizeof .*
只要vtable还在，动态绑定就在，关键在于vtable是谁的
```
Derived d;
Base *b = &d;
(*b).f(); // Call derived f
```

Default parameters in dynamic binding

class Base{
public:
    int x;
    virtual void f(int x = 1)
    {
        cout << "Base" << x << endl;
    };
};

class Derived : public Base{
public:
    void f(int x = 2){
        cout << "Derived" << x << endl;
    }
};

dynamic cast 的源类型必须是polymorphic 否则编译错误

然后才是运行时检查是否能转换。及时转换目标完全不是源类型的派生类，也只是运行时返回NULL指针。