Strings in C++: quotes, characters, and valid indices

"code" is a string literal, while 'c' is one character

The quote marks decide what kind of value you wrote. Double quotes make "code" a string literal containing four characters. Single quotes make 'c' a character literal containing exactly one char. The two forms are not interchangeable, even though both use quote marks.

"code"
'c'

The complete text for the running example needs double quotes, so "code" can be used to initialize a string. Writing 'code' does not turn several letters into a string literal. It is not the single-character form, and it cannot initialize the string variable that holds the complete text.

std::string word = "code" creates one variable holding the complete text

The declaration names the type, the variable, and its initial value in one line. std::string is the type for a sequence of characters, word is the variable name, and "code" is the initial value placed in that variable. The include makes the std::string type available.

#include <iostream>
#include <string>

int main() {
    std::string word = "code";
    std::cout << word << '\n';
}

There is one variable here, not four separate variables. The value stored in word is the complete text code. The characters inside that text can be selected individually, but selecting one character does not split word into separate values.

word[i] selects the character stored at index i

Each character in word has a zero-based index. The first character c is at index 0, not index 1. Moving one position at a time gives o at index 1, d at index 2, and e at index 3. Each expression such as word[2] produces a char, while word itself remains the complete std::string value "code".

std::string word = "code";

std::cout << word[0] << '\n';
std::cout << word[1] << '\n';
std::cout << word[2] << '\n';
std::cout << word[3] << '\n';

word[4] is outside the string and cannot be read safely

For this exact value, the only valid character indices are 0, 1, 2, and 3. Index 4 is beyond the final character e, so there is no stored character at that index. The fact that 4 comes immediately after 3 does not create another valid position.

std::string word = "code";

std::cout << word[3] << '\n';  // valid, selects e
std::cout << word[4] << '\n';  // out-of-bounds access