JuniorTheoryCommon

What does the program print?

C++

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

int main() {
    std::map<std::string, int> m{{"a", 1}};
    int b = m["b"];
    std::cout << b << ' ' << m.size() << '\n';
}

0 2

Expecting 0 1: m["b"] inserts the missing key "b" before it returns the value, so the size grows to 2.
Expecting undefined behavior: the inserted value is value-initialized, which makes an int 0, as in int x{};.

How do you check whether m has the key "b" without inserting it?
Why does m["b"] not compile when m is a const std::map?

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JuniorTheoryCommon

What does the program print?

C++

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

int main() {
    std::map<std::string, int> m{{"a", 1}};
    int b = m["b"];
    std::cout << b << ' ' << m.size() << '\n';
}

0 2

Expecting 0 1: m["b"] inserts the missing key "b" before it returns the value, so the size grows to 2.
Expecting undefined behavior: the inserted value is value-initialized, which makes an int 0, as in int x{};.

How do you check whether m has the key "b" without inserting it?
Why does m["b"] not compile when m is a const std::map?

Back to questions Open topic in roadmap →Answer it yourself — Quiz