C 中的 Const 引用：類型說明符之前還是之後的 Const？

Const Reference: Before vs. After Type-Specifier

In C++, there is a subtle difference in syntax for declaring a const reference:

int foo1(const Fred &arg) {
...
}

int foo2(Fred const &arg) {
...
}

Both declaration methods produce a constant reference to a Fred object. However, the placement of the const keyword has no semantic impact.

As a Matter of Style

While the behavior of these declarations is identical, there are stylistic preferences to consider.

Some prefer const T& (and const T*), citing:

• Alignment with Stroustrup's book and the C++ standard
• Familiarity among experienced C programmers
• Common usage in the industry

Others favor T const& (and T const*), based on readability:

• Consistent right-to-left parsing
• Reduced risk of misplacing the * or interpreting T const* incorrectly

Right-to-Left Parsing

The right-to-left parsing rule states that declarations should be read from right to left. Therefore, const T& can be interpreted as "a reference to a const T" or "a const T reference." Similarly, T const& can be read as "a const reference to a T" or "a T const reference." Both interpretations are grammatically correct, making the placement of the const keyword less impactful on readability than some argue.

Ultimately, the choice between these declaration styles is a matter of personal preference. Both are syntactically correct and convey the same meaning.

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