「const」如何增強您的 C 代碼：綜合指南

Multiple Applications of "const" in C++: A Comprehensive Guide

"const" is a versatile keyword in C++ that offers numerous ways to enhance code clarity, efficiency, and reliability. Its various applications can be categorized as follows:

1. Referencing Temporaries with Extended Lifetime:

By binding temporaries to references-to-const, you can extend their lifetime beyond the scope of the temporary's declaration. Even if the base reference is non-virtual, the appropriate destructor will still be invoked.

Example:

ScopeGuard const& guard = MakeGuard(&cleanUpFunction);

2. Denoting Unchanged Logical State:

Adding "const" to methods indicates that they won't alter the logical state of the object. This helps others understand the method's purpose.

Example:

struct SmartPtr {
    int getCopies() const { return mCopiesMade; }
};

3. Copy-on-Write Classes:

"const" can be used in copy-on-write classes to assist the compiler in determining when copies need to be made.

Example:

struct MyString {
    char* getData() { /* copy: caller might write */ return mData; }
    char const* getData() const { return mData; }
};

In this scenario, shared data can be maintained until one of the objects requires modifications.

4. Enabling Copy Construction from Constants:

Overloading the copy constructor allows for copies to be created from const objects and temporaries.

Example:

struct MyClass {
    MyClass(MyClass const& that) { /* make copy of that */ }
};

5. Creating True Constants:

"const" can be used to define constants that cannot change under any circumstances.

Example:

double const PI = 3.1415;

6. Passing Objects by Reference:

Objects can be passed as references instead of values to avoid expensive or impossible by-value passing.

Example:

void PrintIt(Object const& obj) {
    // ...
}

In summary, "const" provides multiple tools for optimizing code, preventing errors, and enhancing understanding. Mastering its various uses can significantly improve C++ development practices.

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