Can I get a value from std::map without crashing if the key doesn't exist?

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?

Your program shouldn't usually crash. Do you use map<K,V*> by any chance? Also note that map::find returns an iterator, you can happily use the value notion of the iterator (second) — Zeta
– Zeta, Commented Nov 24, 2013 at 22:38
Yes the map is std::map<std::string, rtobject_t*> rtstructure; where rtobject_t is a struct. — Baldur Gíslason
– Baldur Gíslason, Commented Nov 25, 2013 at 10:37
So apparently, the [] operator returns a new uninitialized pointer if referring to a member that doesn't exist. — Baldur Gíslason
– Baldur Gíslason, Commented Nov 25, 2013 at 16:59

Bryan Chen · Accepted Answer · 2013-11-24 22:38:58Z

5

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;
}

answered Nov 24, 2013 at 22:38

Bryan Chen

46.7k19 gold badges119 silver badges148 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

Can I get a value from std::map without crashing if the key doesn't exist?

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related