This is a common pattern I use to index tokens as they come in: check if the token is in a map, and if not, add it to the map, assigning the map's size.

When doing this in C++, it unexpectedly increments the map's size before the assignment is made:

#include <cstdio>
#include <map>
using namespace std;

int main() {
    map<char, int> m;
    printf("Size before adding: %d\n", m.size());
    m['A'] = m.size();
    printf("Size after adding: %d\n", m.size());
    printf("What was added: %d\n", m['A']);

    return 0;
}

This prints out:

Size before adding: 0
Size after adding: 1
What was added: 1

The way I understand it, it should evaluate the right hand side, which is zero, then pass it to a function that puts 'A' and the zero into the map. But it seems to be evaluating it after it started assigning, which doesn't make sense.

Shouldn't the right-hand side be evaluated before the assignment operation?

c++ dictionary assignment-operator
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