How Does `const` Linkage Differ Between C and C ?

Const and Internal Linkage in C vs. C

Within the realm of C programming, the concept of "const" carries implications for linkage behavior that differ significantly from its counterpart in C. This discrepancy can raise questions for developers.

In C , when a variable is declared with the "const" keyword at namespace scope, it acquires internal linkage. This implies that the variable is only visible within the compilation unit in which it is defined. However, in C, a comparable construct with "const" and file scope yields external linkage, making the variable accessible across multiple compilation units.

To illustrate this difference, consider the following code snippet in C :

const int var_a = 1;
int var_b = 1;

Upon compilation with g -c test.cpp, only var_b is exported, while var_a remains secluded. This is because var_a has internal linkage due to its "const" declaration at namespace scope.

The rationale behind this divergence lies in the differing use cases of "const" in C and C. In C , the "const" keyword serves not only to indicate a constant value but also enables the usage of compile-time values. By associating internal linkage with "const", C encourages explicit initialization values and facilitates the inclusion of "const" objects in header files used across multiple compilation units. Thus, "const" plays a crucial role in fostering encapsulation and modularity in C code.

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