Program crashes on trying to access the key that does not exist in map

Question

I am doing a simple program that tries to fetch a non-existent key-value pair in the map. But when I run the code, I get a runtime error saying ==8406==WARNING: AddressSanitizer failed to allocate 0x7ffe7f82c881 bytes ==8406==AddressSanitizer's allocator is terminating the process instead of returning 0 ==8406==If you don't like this behavior set allocator_may_return_null=1

#include <iostream>
#include <map>
#include <string>

int main ()
{
    typedef std::map<std::string,int> mymap;
    mymap cnt;

    cnt["a"]=10;

    auto f = first.find("asdasd");
    std::cout << f->second;
    return 0;
}

at line where I do f->second. How could I handle the case where the key does not exist in the map?

"How could I handle the case where the key does not exist in the map" - Simple. Just check for `map::end`. — Jesper Juhl, Apr 11 '20 at 17:21

score 3 · Answer 1 · answered Apr 11 '20 at 17:15

3

Assuming you mean

auto f = cnt.find("asdasd");

you need to check if the key that you looked for actually exists

if (f != cnt.end())
    std::cout << f->second;

Otherwise you are dereferencing an invalid location, and that's UB.

answered Apr 11 '20 at 17:15

cigien

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score 3 · Accepted Answer · answered Apr 11 '20 at 17:16

3

The find member function of std::map specializations returns a past-the-end iterator if the key does not exist.

So you need to compare to the container's end():

 auto f = cnt.find("asdasd");
 if (f != cnt.end()) {
     std::cout << f->second;
 }

answered Apr 11 '20 at 17:16

aschepler

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Program crashes on trying to access the key that does not exist in map

2 Answers2