How is the reserve function used in C++?

In C++, the reserve function is used to allocate a certain amount of memory space in advance so that the container does not need to reallocate memory frequently when adding elements, thereby improving performance. The reserve function is typically used before adding a large number of elements to a container to allocate a certain amount of memory space.

For example, when dealing with a std::vector container, you can use the reserve function to allocate a certain amount of memory space in advance.

std::vector<int> vec;
vec.reserve(100); // 预留100个元素的内存空间

// 向vec中添加元素
for (int i = 0; i < 100; ++i) {
    vec.push_back(i);
}

In the above example, memory space for 100 elements was reserved by calling the reserve function, and then 100 elements were added to the vec. This can avoid frequent memory allocation when adding elements, thereby improving the performance of the program.