C++指针编程你要小心的陷阱——如何优雅的释放指针void*

指针操作是C++开发中必备技能。尽管C++11开始引入了智能指针以缓解普通指针的滥用，但是某些场合必须使用普通指针。释放指针在C/C++编程中非常重要，一般推荐释放指针后立即将指针设置为null，防止出现低级的野指针问题（只能避免低级别的野指针）同时方便调试。

一、C语言时代

在C语言编程中，我们由于没有C++模板，函数重载功能，所以一般定义一个统一的宏来用于释放指针。

// 删除指针  
#define SAFE_DELETE(p) { \  
    if (NULL != (p)) { \  
        free((p)); \  
        (p) = NULL;\  
    }\  
}

二、C++时代

C++相对C语言的改进就是引入了面向对象操作，支持函数重载、类继承、模板、异常处理等等概念。在C++中，一般用函数模板来操作释放指针，这样的好处是可以进行类型检查。

// 删除数组
template <typename T>
inline void safe_delete(T *&target) {
    if (nullptr != target) {
        delete target;
        target = nullptr;
    }
}

// 删除数组指针
template <typename T>
inline void safe_delete_arr(T *&target) {
    if (nullptr != target) {
        delete[] target;
        target = nullptr;
    }
}

三、void *指针问题

在C、C++ 中，void * 指针可以转换为任意类型的指针类型，在删除void*指针时编译器往往会发出如下警告
warning: deleting 'void*' is undefined [enabled by default]
翻译：警告：删除“void *”指针可能引发未知情况（默认打开警告）
永远记住，在C、C++开发中绝对不能忽视警告，一定要重视警告，最好消除警告。有些警告无关紧要，有些警告却是bug的根源；删除void *指针的警告就属于后面一种情况，可能引起严重的bug而且难以发现：

1. 使用delete pointer; 释放void指针void *，系统会以释放普通指针（char, short, int, long, long long）的方式来释放void *指向的内存空间；
2. 如果void *指向一个数组指针，那么由于释放指针时用了delete pointer从而导致内存泄漏，释放指针正确做法是delete[] pointer;
3. 如果void *指向一个class类，那么系统由于认为void *指向一个普通的内存空间，所以释放指针时系统class的析构函数不会调用；

释放void *的解决方案：将void *转换为原来类型的指针，然后再调用delete释放指针，如果原来的指针是数组指针，那么必须使用delete []删除指向的内存空间。
在C++中我们可以使用模板定义内联函数：

template <typename T>
inline void safe_delete_void_ptr(void *&target) {
	if (nullptr != target) {
		T* temp = static_cast<T*>(target);
		delete temp;
		temp = nullptr;
		target = nullptr;
	}
}

调用方法

int *psample = new int(100);
safe_delete_void_ptr<int>(psample);

利用模板实例化参数统一简化过程。
测试代码
safe_delete_demo.cpp

#include <cstddef>
#include <cstdlib>
#include <string>
#include <iostream>

template <typename T>
inline void safe_delete(T *&target) {
    if (nullptr != target) {
        delete target;
        target = nullptr;
    }
}

template <typename T>
inline void safe_delete_void_ptr(void *&target) {
    if (nullptr != target) {
        T* temp = static_cast<T*>(target);
        delete temp;
        temp = nullptr;
        target = nullptr;
    }
}

class A {
public:
    A(std::string name) {
        this->name = name;
    };
    virtual ~A() {
        std::cout<<"base class A's destructor"<<", name: "<<this->name<<std::endl;
    };
public:
    std::string name;
};

class AChild: public A {
public:
    AChild(std::string name, std::string school)
    : A(name){
        this->school = school;
    };

    ~AChild() {
        std::cout<<"child class AChild's destructor"<<", name: "<<this->name
            <<", school: "<<this->school<<std::endl;
    };

public:
    std::string school;
};

int main(int argc, char *argv[]) {
	// 测试safe_delete释放普通类指针
    std::cout<<"safe_delete pointer of type AChild"<<std::endl;
    AChild *a1 = new AChild("jacky", "Shenzhen University");
    safe_delete(a1);
    std::cout<<std::endl;

	// 测试safe_delete释放void*指针
    std::cout<<"safe_delete pointer of type void *"<<std::endl;
    void *vp = new AChild("Polyn", "Southern University of Science and Technology");
    safe_delete(vp);
    std::cout<<std::endl;

	// 测试safe_delete_void_ptr释放模板实例化为基类的void*指针
    std::cout<<"safe_delete_void_ptr pointer of type void * ==> A *"<<std::endl;
    void *vpA = new AChild("Heral", "Renmin University of China");
    safe_delete_void_ptr<A>(vpA);
    std::cout<<std::endl;

	// 测试safe_delete_void_ptr释放模板实例化为子类的void*指针
    std::cout<<"safe_delete_void_ptr pointer of type void * ==> AChild *"<<std::endl;
    void *vpAChild = new AChild("pevly", "Southeast University");
    safe_delete_void_ptr<AChild>(vpAChild);
    return 0;
}

编译及运行

$ g++ -std=c++11 safe_delete_demo.cpp
safe_delete_demo.cpp: In instantiation of 'void safe_delete(T*&) [with T = void]':
safe_delete_demo.cpp:59:16: required from here
safe_delete_demo.cpp:9:9: warning: deleting 'void*' is undefined [enabled by default]
delete target;

$ ./a.out
safe_delete pointer of type AChild
child class AChild's destructor, name: jacky, school: Shenzhen University
base class A's destructor, name: jacky

safe_delete pointer of type void *
safe_delete_void_ptr pointer of type void * ==> A *
child class AChild's destructor, name: Heral, school: Renmin University of China
base class A's destructor, name: Heral

safe_delete_void_ptr pointer of type void * ==> AChild *
child class AChild's destructor, name: pevly, school: Southeast University
base class A's destructor, name: pevly

通过测试用例我们可以看出。

1. 使用safe_delete释放明确的类会自动触发析构函数（如果析构函数为虚函数，那么先调用子类的析构函数再调用子类的直接基类的析构函数）；
2. 使用safe_delete释放void*指针指向的类时，不会触发对应类的析构函数；
3. 如果使用safe_delete_void_ptr内联函数释放void*指针，那么由于在释放指针前，函数会将void*指针转换为特定类型的函数指针，所以最终能够触发调用析构函数，并且不影响虚类的释放行为。

C++指针编程你要小心的陷阱——如何优雅的释放指针void*

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