我有一个 X 类,我想将它放入 std::map 类型的 STL 映射中.STL 映射需要将 X 存储在内存中的某处,因此我正在寻找一种有效的(运行时和内存)方法来创建 X 并将其存储在映射中.
I have a class X that I would like to put into an STL map of type std::map. An STL map needs to have X stored in memory somewhere so I'm looking for an efficient (run time and memory) way to create X and store it in the map.
我注意到以下代码,其中 x 是 X 类型的对象,stlMap 是 std::map 类型的映射:
I noticed that the following code where x is an object of type X and stlMap is a map of type std::map:
stlMap["test"] = x;调用以下结果:
为什么要创建这么多 X 对象?
Why are so many X objects being created?
时间和内存的使用效率低下吗?
Is it an inefficient use of time and memory?
有没有更好的方法将对象放入地图中?也许将映射更改为字符串映射到 x*?
Is there a better way to put an object into a map? Maybe changing the map to be a map of strings to x*?
推荐答案试试 stlMap.insert( map::value_type("test", x) ):
#include <iostream> #include <string> #include <map> using namespace std; class X { public: X() { cout << "X default constructor" << endl; } ~X() { cout << "X destructor" << endl; } X( const X& other ) { cout << "X copy constructor" << endl; } X& operator=( const X& other ) { cout << "X copy-assignment operator" << endl; } int x; }; int main() { X x; map< string, X > stlMap; cout << "INSERT BEGIN" << endl; stlMap.insert( map< string, X >::value_type( "test", x ) ); cout << "INSERT END" << endl; stlMap.clear(); cout << "ASSIGN BEGIN" << endl; stlMap["test"] = x; cout << "ASSIGN END" << endl; return 0; }在我的 g++ 上,将事情简化为:
On my g++ that whittles things down to:
根据 ArunSaha 的建议,更新了测试.insert() 输出不变,而赋值序列如下所示:
Per ArunSaha's suggestion, updated the test. The insert() output is unchanged, while the assignment sequence looks like this:
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STL 映射值构造函数
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