std::map 索引和插入调用之间的差异

发布于 2024-08-08 12:42:20 字数 197 浏览 2 评论 0 原文

std::map 的索引重载运算符和 insert 方法调用有什么区别？

即：

some_map["x"] = 500;

vs.

some_map.insert(pair<std::string, int>("x", 500));

原文

What is the difference between the index overloaded operator and the insert method call for std::map?

ie:

some_map["x"] = 500;

vs.

some_map.insert(pair<std::string, int>("x", 500));

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抠脚大汉 2024-08-15 12:42:20

我相信 insert() 不会覆盖现有值，并且可以通过测试返回的迭代器/对值中的 bool 值来检查操作的结果。

对下标运算符 [] 的赋值只会覆盖那里的任何内容（如果插入一个条目。

如果您不期望这种行为并且不适应它，则插入和 [] 运算符中的任何一个都可能会导致问题

例如 with insert:

std::map< int, std::string* > intMap;
std::string* s1 = new std::string;
std::string* s2 = new std::string;
intMap.insert( std::make_pair( 100, s1 ) ); // inserted
intMap.insert( std::make_pair( 100, s2 ) ); // fails, s2 not in map, could leak if not tidied up

和 with [] 运算符：

std::map< int, std::string* > intMap;
std::string* s1 = new std::string;
std::string* s2 = new std::string;
intMap[ 100 ] = s1; // inserted
intMap[ 100 ] = s2; // inserted, s1 now dropped from map, could leak if not tidied up

我认为这些是正确的，但还没有编译它们，所以可能有语法错误

I believe insert() will not overwrite an existing value, and the result of the operation can be checked by testing the bool value in the iterator/pair value returned

The assignment to the subscript operator [] just overwrites whatever's there (inserting an entry if there isn't one there already)

Either of the insert and [] operators can cause issues if you're not expecting that behaviour and don't accommodate for it.

Eg with insert:

std::map< int, std::string* > intMap;
std::string* s1 = new std::string;
std::string* s2 = new std::string;
intMap.insert( std::make_pair( 100, s1 ) ); // inserted
intMap.insert( std::make_pair( 100, s2 ) ); // fails, s2 not in map, could leak if not tidied up

and with [] operator:

std::map< int, std::string* > intMap;
std::string* s1 = new std::string;
std::string* s2 = new std::string;
intMap[ 100 ] = s1; // inserted
intMap[ 100 ] = s2; // inserted, s1 now dropped from map, could leak if not tidied up

I think those are correct, but haven't compiled them, so may have syntax errors

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