Question

How would I access elements from the following map:

map<int, string[4]> * my_map;

I used to do it through the at() operator

string * val_ptr = my_map->at(key);

Recently, I have discovered that this is a non standard feature of my compiler and the correct way of doing it is through operator[]. Unfortunately, the compiler keeps trying to convert my key to string [4]:

string * val_ptr = my_map->operator[](key);

error: conversion from ‘int’ to non-scalar type ‘std::string [4]’ requested

I have looked online, but there don’t seem to be any examples with a map of string arrays. Am I doing something invalid? Should I be using a vector instead, and if so, would it be slower to create and access?

Answer 1

Use of .at() function is not non-Standard anymore. It is in the Standard C+11 (see doc).

Now, this,

string * val_ptr = my_map->operator[](key);

which is correct, but it should be written as:

string * val_ptr = (*my_map)[key];

as it is more succinct.

As for the compiler error, it is somewhere else.

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In fact, I believe, the problem is coming from somewhere, and caused by pointer declaration of the map. Why don’t you declare the map as:

map<int, string[4]> my_map; //no pointer

and then use

string * val_ptr = my_map[key]; 

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Even better if you use std::vector:

std::map<int, std::vector<std::string> > my_map; //no pointer

and then use

std::vector<std::string> & val = my_map[key];