When Concatenating String Literals with \' \' in C , Why Can\'t They Be Directly Joined?

Concatenating String Literals

In Accelerated C , Koenig introduces the ability to concatenate strings using the " " operator. While this works with a combination of strings and string literals, it raises questions when attempting to concatenate multiple string literals.

Understanding the Issue

Consider the following example:

const string hello = "Hello";
const string message = hello + ",world" + "!";

This code executes successfully, despite the presence of ",world" and "!" as string literals. However, a similar attempt fails with the following:

const string exclam = "!";
const string message = "Hello" + ",world" + exclam;

The Associativity Factor

The difference lies in the associativity of the " " operator. It evaluates from left to right, meaning:

(hello + ",world") + "!"

In the first case, "hello" is concatenated with ",world," resulting in a string object. This object is then concatenated with "!".

In the second case, "Hello" and ",world" cannot be directly concatenated due to the restriction against concatenating two string literals. This results in a compiler error.

Possible Solutions

To resolve the issue, one can either ensure that the first two operands to " " are string objects or force the evaluation order using parentheses:

Option 1: String Object as First Operand

const string message = string("Hello") + ",world" + exclam;

Option 2: Parentheses

const string message = "Hello" + (",world" + exclam);

Why String Literals Cannot Be Concatenated Directly

String literals are arrays of characters, and when used in an expression, they convert to pointers to their initial elements. Attempting to add two string literals is equivalent to adding two pointers, which is not valid.

Alternative for Concatenating String Literals

String literals can be concatenated simply by placing them adjacent to each other:

"Hello" ",world"

This is equivalent to:

"Hello,world"

However, this approach only works with string literals and not with string objects or character arrays.

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