C++ capture error "throwing an instance of 'std::out_of_range'"

Question

This will be my 1st question here. I have came across an issue that I would like to resolve in a program that I'm working on (doing some intern/developer shadowing tasks).

I have a std::map data structure. Every time I pass from the cmd line a value that is not mapped I get the: terminate called after throwing an instance of 'std::out_of_range' what(): map::at Aborted (core dumped)

And it is fine, this is the way it should be. However what I would like to achieve is to validate the user input and if the value is not present in the map then for example capture the above event in a variable and then print it out.

Every conditional statement that I try is not working as I would like to as it instantly throws out that "terminate called..." ad closes down the whole program.

Is getting input and checking if the key is already in the map an ok solution? https://stackoverflow.com/questions/1939953/how-to-find-if-a-given-key-exists-in-a-c-stdmap — GustavD, Aug 11 '21 at 09:05
Sorry but as mentioned. Tried to do it with if statements and it was not working and only throwing that terminate called. — FrankH, Aug 11 '21 at 10:46

score 0 · Accepted Answer · answered Aug 11 '21 at 09:15

you should surround your std::map::at(key) call inside a try/catch block as in

std::map<std::string, int> my_map = {{"one",1}, {"two":2}};

try
 { 
  auto val = my_map.at("three");
  std::cout << "my value at 'three' is " << val << "\n"; 
 } 
catch(std::out_of_range const & e)
 { 
  std::cerr << "oops 'three' is not in your map\n";
 }

score 0 · Answer 2 · answered Aug 11 '21 at 09:39

You can catch the error with a try-catch block but you can easily avoid throwing it in the first place.

In c++20 you can use std::map.contains(key_type) to check whether it is in the map.

Without c++20 you can use std::map.find(key_type) and then assert that it is not equal to std::map.end().

Example:

std::map<int,int> a = {{1, 5}, {2, 3}};
if(a.contains(3))
    // Do something
else
    // Do something else

auto it = a.find(2)
if(it != a.end()) // Meaning that value was found
    // Do something
else
    // Do something else

C++ capture error "throwing an instance of 'std::out_of_range'"

2 Answers2