How Can I Concatenate Strings Using C/C Macros?

Concatenating Strings with C/C Macros

In C and C , macros provide a convenient way to define constants and perform text substitutions during pre-processing. Suppose you have two macros, STR1 and STR2, defined as follows:

#define STR1      "s"
#define STR2      "1"

You want to concatenate STR1 and STR2 to obtain "s1". Is there a straightforward way to do this with a single macro that produces the desired string?

A Direct Approach

Unlike tokens in code, macros are processed by the preprocessor before compilation. Therefore, you can't use operators like ' ' to concatenate strings directly with macros. However, depending on the specific syntax, it may be possible to achieve this.

In this case, C allows you to concatenate string literals separated by whitespace during macro expansion. By modifying the STR3 definition as follows, you can achieve the desired result:

#define STR3 STR1 STR2

This expands to:

#define STR3 "s" "1"

In C, separating two strings with whitespace is equivalent to creating a single string. Therefore, STR3 expands to the string "s1".

Alternative Approaches

If the direct approach does not work in your scenario, alternative methods include using preprocessor conditionals or passing arguments to a parent macro that performs the concatenation. These approaches may be more complex but provide more flexibility in different situations.

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