How to improve C++ template efficiency?

Tips to improve C++ template efficiency: Avoid full specialization, use partial specialization or explicit instantiation instead. Use constexpr functions and data members for compile-time calculations. Inline template functions and classes to reduce function call overhead. Optimize type erasure, either by explicit instantiation or using std::enable_if. Cache template calculations to avoid double calculations.

Tips to Improve C++ Template Efficiency

C++ templates are essential for creating common, reusable code, but sometimes they can be less efficient than expected . Here are some tips for making C++ templates more efficient:

Avoid full specialization

Full specialization leads to code bloat because the compiler must generate a separate instance for each specific type. Try to use partial specialization or explicit instantiation to generate only the necessary code.

// 部分特化
template <typename T>
struct MyType {
    // ...
};

template <>
struct MyType<int> {
    // 特殊化实现
};

Using constexpr functions and data members

constexpr functions and data members allow results to be calculated at compile time, thus avoiding unnecessary overhead at runtime.

// constexpr 函数
constexpr int my_function(int x) {
    return x * x;
}

// constexpr 数据成员
constexpr int my_data = my_function(42);

Inline template functions and classes

Inline template functions and classes insert code directly into the call site, thereby reducing the overhead of function calls.

// 内联模板函数
template <typename T>
inline void my_function(T&& x) {
    // ...
}

// 内联模板类
template <typename T>
inline class MyClass {
    // ...
};

Practical case

Case 1: Optimization type erasure

Type erasure refers to hiding template parameter type information at runtime. This can cause a performance penalty because it forces the compiler to insert additional code to retrieve type information.

We can avoid type erasure by explicitly instantiating a specific type or using std::enable_if. The following example demonstrates how to optimize type erasure in a std::vector container:

// 显式实例化
std::vector<int> my_vector;

// 使用 std::enable_if
template <typename T, typename = std::enable_if_t<std::is_integral<T>::value>>
std::vector<T> my_vector;

Case 2: Cache template calculation

Template function and classes often perform complex calculations or lookup operations. To improve efficiency, we can cache these calculation results in local variables or static member variables.

// 模板函数缓存
template <typename T>
int my_function(T&& x) {
    static int cache[100];
    if (cache[x] == 0) {
        cache[x] = std::complex_calculation(x);
    }
    return cache[x];
}

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