Why Does Top-Level Const on Non-Class Return Types Seem Redundant?
Nov 04, 2024 am 07:56 AMDo Non-Class Return Types Benefit from Const?
In C , the usage of const has been emphasized as a best practice for immutable data handling. However, it may seem that qualifying non-class return types with const yields no noticeable difference:
<code class="cpp">int foo() { } const int foo() { }</code>
Why, then, does this distinction exist, as exemplified by the Bigint class:
<code class="cpp">int& operator[](const int index); const int operator[](const int index) const;</code>
Explanation:
Top-level const qualifiers applied to the return types of non-class functions are effectively ignored, rendering the two declarations above identical. However, this is not the case for references:
<code class="cpp">int& operator[](int index); int const& operator[](int index) const;</code>
In such scenarios, the distinction is meaningful.
Furthermore, top-level const qualifiers on return types are similarly ignored in function declarations.
The relevance of const qualifiers also extends to class return types. If a function returns T const, any attempt to invoke non-const functions on the returned object will result in an error, as illustrated below:
<code class="cpp">class Test { public: void f(); void g() const; }; Test ff(); Test const gg(); ff().f(); // legal ff().g(); // legal gg().f(); // illegal gg().g(); // legal</code>
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