Question

I’m from php and when I define something like this:

$a = array();
echo $a[586];

I will get a notice about undefined index.

Meanwhile at c++ I can do this:

map<int, string> my_map;
cout << "Map is: " << my_map[34535];

string sentence = "acdefb s";
cout << "Letter is: " << sentence[15];

And I won’t get any error.

Why it’s not possible at php and possible at c++?

Answer 1

Because PHP and C++ are two radically different languages with radically different semantics. When using the [] operator with an std::map object in C++, a new element is inserted into the map if the key does not already exist within the map. In PHP, this is not the case.

As for your second example with std::string sentance, it is not valid to use the [] operator with an invalid index (an index which is >= sentance.length()) on an std::string in C++. You may not get any “error”, because the result of using an invalid index with std::string is undefined behavior, meaning that anything can happen. One common result on modern platforms is a Segmentation Fault/Access Violation, which generally will crash your program.

With std::string you can use std::string::at() instead of the [] operator if you want protection against undefined behavior. The at function does a bounds check and throws std::out_of_range if you use an invalid index.