How to perform memory optimization when using C++ STL?

Use the following optimization strategies to optimize memory usage in C++ STL: 1. Use a custom allocator to control the memory allocation method; 2. Use reserve() to pre-allocate space to avoid dynamic memory allocation overhead; 3. Use move semantics or Reference semantics to avoid unnecessary memory copies.

Memory Optimization in C++ STL

STL (Standard Template Library) is a widely used library in C++ that provides A set of efficient and well-tested data structures and algorithms. However, when using STL, improper memory management can cause performance issues. Here are some tips for optimizing memory usage:

Using custom allocators

You can control how an STL container allocates memory by providing a custom allocator. Custom allocators can implement various optimization strategies, such as:

// 自定义分配器用于使用内存池分配内存
class MyAllocator {
    std::vector<int> memory_pool;
public:
    void* allocate(std::size_t size) {
        if (memory_pool.size() >= size) {
            void* ptr = &memory_pool[0];
            memory_pool.erase(memory_pool.begin());
            return ptr;
        }
        return std::malloc(size);
    }
    void deallocate(void* ptr, std::size_t size) {
        // 将内存返回到池中
        memory_pool.push_back(*static_cast<int*>(ptr));
    }
};

By passing MyAllocator to the container constructor, we can use custom allocation strategies:

std::vector<int, MyAllocator> my_vector;

Using container size optimization

STL containers often use dynamic memory allocation, so it is critical to pre-allocate enough space. A given number of elements can be preallocated using the reserve() method:

std::vector<int> my_vector;
my_vector.reserve(100);

Avoid unnecessary copying

STL algorithms and container operations can Creates new objects, causing unnecessary memory copying. To avoid this situation, you can use move semantics or reference semantics. For example, use std::move() to move elements to a container instead of copying:

std::vector<int> my_vector;
my_vector.push_back(std::move(my_value));

Practical case

The following example demonstrates Learn how to optimize memory allocation using a custom allocator:

#include 
#include 

// 自定义分配器使用内存池分配内存
class MyAllocator : public std::allocator {
    std::vector memory_pool;
public:
    MyAllocator() {}
    MyAllocator(const MyAllocator&) = default;
    template
    MyAllocator(const MyAllocator&) {}
    int* allocate(std::size_t n) {
        if (n <= memory_pool.size()) {
            int* ptr = &memory_pool[0];
            memory_pool.erase(memory_pool.begin());
            return ptr;
        }
        return std::allocator::allocate(n);
    }
    void deallocate(int* ptr, std::size_t) {
        // 将内存返回到池中
        memory_pool.push_back(*ptr);
        std::allocator::deallocate(ptr, 1);
    }
};

int main() {
    // 使用自定义分配器创建 vector
    std::vector<int, MyAllocator> my_vector;

    // 分配 1000 个元素
    my_vector.reserve(1000);

    // 使用自定义分配器分配的内存的效率更高
    return 0;
}

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