Const Reference in C : `const T&` or `T const&` – What's the Difference?

Const Reference Types in C : Placement of the 'const' Qualifier

In C , there are two ways to declare constant references using the 'const' qualifier:

const Fred &arg;
Fred const &arg;

Semantic Differences

Semantically, these two declarations are equivalent, and the language considers them as the same type. There is no functional difference between passing an argument by const T& or T const&.

Style Considerations

However, when it comes to style, there are different preferences among programmers.

Left-to-Right Parsing

Some programmers advocate for placing the 'const' qualifier after the type (T const&), arguing that it follows the right-to-left parsing rule in C . This ensures that when reading the declaration, the 'const' qualifier is applied to the type, rather than to the reference itself.

Right-to-Left Parsing

Others, however, prefer the const T& syntax, arguing that it reads equally well right-to-left. It can be interpreted as "reference to a T constant". Furthermore, placing the 'const' qualifier before the reference prevents the accidental declaration of a pointer constant, such as 'T* const' which is illegal.

Common Practice

In practice, both styles are widely used. The style preferred by Stroustrup's "The C Programming Language" and the C standard is const T&. However, the style used in K&R's "The C Programming Language" and the C standard is T const*. Ultimately, the choice of style is a matter of personal preference and may also be influenced by the coding conventions of specific organizations.

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