C/C++编程：确保容器中的对象拷贝正确而高效

分手后的思念是犯贱 2023-01-12 11:41 120阅读 0赞

> 容器中保存了对象，但并不是你提供给容器的那些对象。而当你从容器中取出一个对象时，你所取出的也不是容器中所保存的那份。当向容器中加入对象时，存入容器的是你所指定对象的拷贝。当从容器中取出一个对象时，你所得到的是容器中所保存的对象的拷贝。进去的是拷贝，出来的也是拷贝，也就是STL的工作方式。

## vector ##

class read_hook{
    public:
    
        void *ctx;
        char disable;
    };
    
    int main(){
        std::vector<read_hook*> vec;
    
        for (int i = 0; i < 3; ++i) {
            read_hook *hook = new read_hook;
            printf("%p\t", hook);
            vec.push_back(hook);
        }
        printf("\n");
    
        for (int i = 0; i < 3; ++i) {
            read_hook *hook = vec[i];
            printf("%p\t", hook);
        }
        printf("\n");
    
        for(auto iter : vec){
            read_hook *hook = iter;
            printf("%p\t", hook);
        }
    
        printf("\n");
        for(auto &iter : vec){
            read_hook *hook = iter;
            printf("%p\t", hook);
        }
     
        __gnu_cxx::__normal_iterator<read_hook **, std::vector<read_hook *>> iter;
        for (iter = vec.begin(); iter != vec.end(); ++iter) {
            read_hook *hook = *iter;
            printf("%p\t", hook);
        }
        printf("\n");
    }

![watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBAT2NlYW4mJlN0YXI_size_9_color_FFFFFF_t_70_g_se_x_16][]

好的写法：

class read_hook{
    public:
        read_hook(){
            printf("read_hook, %p\n", this);
        }
        ~read_hook(){
            printf("~read_hook, %p\n", this);
        }
        void *ctx;
        char disable;
    };
    
    int main(){
        std::vector<read_hook*> vec;
        printf("\n   --- vec.size: %zu --- \n", vec.size());
        for (int i = 0; i < 3; ++i) {
            read_hook *hook = new read_hook;
            vec.push_back(hook);
        }
        printf("\n   --- vec.size: %zu --- \n", vec.size());
    
    
        while (!vec.empty()){
            read_hook *hook = *vec.begin();
            //read_hook *hook = vec.front();
            vec.erase(vec.begin());
            delete hook;
        }
        printf("\n   --- vec.size: %zu --- \n", vec.size());
    
        getchar();
    }

![watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBAT2NlYW4mJlN0YXI_size_7_color_FFFFFF_t_70_g_se_x_16][]

class read_hook{
    public:
        read_hook(){
            printf("read_hook, %p\n", this);
        }
        ~read_hook(){
            printf("~read_hook, %p\n", this);
        }
        void *ctx;
        char disable;
    };
    
    int main(){
        std::vector<read_hook*> vec;
        printf("\n   --- vec.size: %zu --- \n", vec.size());
        for (int i = 0; i < 3; ++i) {
            read_hook *hook = new read_hook;
            vec.push_back(hook);
        }
        printf("\n   --- vec.size: %zu --- \n", vec.size());
    
    
        while (!vec.empty()){
            read_hook *hook = vec.back();
            vec.erase(vec.end() - 1);
            delete hook;
        }
        printf("\n   --- vec.size: %zu --- \n", vec.size());
    
        getchar();
    }

![watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBAT2NlYW4mJlN0YXI_size_6_color_FFFFFF_t_70_g_se_x_16][]

class read_hook{
    public:
        read_hook(){
            printf("read_hook, %p\n", this);
        }
        ~read_hook(){
            printf("~read_hook, %p\n", this);
        }
        void *ctx;
        char disable;
    };
    
    
    int main(){
        std::vector<read_hook*> vec;
        printf("\n   --- vec.size: %zu --- \n", vec.size());
        for (int i = 0; i < 3; ++i) {
            read_hook *hook = new read_hook;
            vec.push_back(hook);
        }
        printf("\n   --- vec.size: %zu --- \n", vec.size());
    
    
        read_hook *hook = vec[1];
        delete hook;
        vec.erase(vec.begin() + 1);
        printf("\n   --- vec.size: %zu --- \n", vec.size());
    
        getchar();
    }

![watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBAT2NlYW4mJlN0YXI_size_6_color_FFFFFF_t_70_g_se_x_16 1][]

不好的写法：

class read_hook{
    public:
        read_hook(){
            printf("read_hook, %p\n", this);
        }
        ~read_hook(){
            printf("~read_hook, %p\n", this);
        }
        void *ctx;
        char disable;
    };
    
    int main(){
        std::vector<read_hook*> vec;
        printf("\n   --- vec.size: %zu --- \n", vec.size());
        for (int i = 0; i < 3; ++i) {
            read_hook *hook = new read_hook;
            vec.push_back(hook);
        }
        printf("\n   --- vec.size: %zu --- \n", vec.size());
        
        for (int i = (int)vec.size(); i > -1; --i) {
            read_hook *hook = vec[i];
            vec.pop_back();
            delete hook;
        }
        printf("\n   --- vec.size: %zu --- \n", vec.size());
    
        getchar();
    }

![watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBAT2NlYW4mJlN0YXI_size_12_color_FFFFFF_t_70_g_se_x_16][]

## list ##

class read_hook {
    public:
        read_hook() {
            printf("read_hook, %p\n", this);
        }
    
        ~read_hook() {
            printf("~read_hook, %p\n", this);
        }
    
        void *ctx;
        char disable;
    };
    
    int main(){
        // list不能随意访问元素
        std::list<read_hook*> list;
    
        for (int i = 0; i < 3; ++i) {
            read_hook *hook = new read_hook;
            list.push_back(hook);
        }
        printf("\n   --- list.size: %zu --- \n", list.size());
    
        while (!list.empty()){
            read_hook *hook = list.back();
            delete hook;
            list.remove(hook);
        }
        printf("\n   --- list.size: %zu --- \n", list.size());
        getchar();
    }

![watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBAT2NlYW4mJlN0YXI_size_7_color_FFFFFF_t_70_g_se_x_16 1][]

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[watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBAT2NlYW4mJlN0YXI_size_12_color_FFFFFF_t_70_g_se_x_16]: /images/20221119/f5c319b1458140dc8e3e483b833eb68b.png
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