如果键不存在，我可以从std :: map获取一个值而不会崩溃吗？

Question

I have C++ code like this:

if(rtstructure.find(varName) != rtstructure.end()) {
    rtdef = rtstructure[varName];
}

where rtstructure is std::map with std::string for the key. This code works but it seems like a waste to make it search twice for the same key. If I omit the if case around the assignment, the program crashes if varName points to a key that doesn't exist.

Can I in a single map operation look up a key in a std::map and get its value if it exists, without crashing if it doesn't exist?

Answer 1

find give you a std::map<>::iterator that holds/point to std::pair<> . The iterator can be saved and reused (given that you did not do anything to invalidate it such as erase ).

// i don't know the type of rtstructure so i use auto
// you can replace it to the correct type if C++11 is not available
auto it = rtstructure.find(varName); 
if(it != rtstructure.end()) {
    rtdef = it->second;
}