stl vector使用之resize和reserve的区别

void resize (size_type n);

void resize (size_type n, const value_type& val);

调整容器大小，使其包含n个元素。

1，如果n小于等于当前容器大小，则容器缩小为前n个元素，删除后面的元素。

#include 
#include 
#include 

int main()
{
    std::vector test;
    test.push_back(1);
    test.push_back(11);
    test.push_back(12);
    test.push_back(13);

    test.resize(2); //容器大小没变，元素减少了
    
    printf("test.capacity = %lu, size = %lun", test.capacity(), test.size()); //capacity = 4, size = 0
    for(unsigned int i = 0; i < test.size(); i++)
    {
        printf("%dn", test.at(i));
    }
    return 0;
}

 2，如果n大于当前容器大小（会引起内存重新分配），则容器扩展内容，在容器的末尾插入足够多的元素以达到n的大小。如果指定了val，则将新元素初始化为val的副本，否则，则对新元素进行值初始化。

#include 
#include 
#include 

int main()
{
    std::vector test;
    test.push_back(1);
    test.push_back(11);
    test.push_back(12);
    test.push_back(13);

    test.resize(6); //容器大小改变
    
    printf("test.capacity = %lu, size = %lun", test.capacity(), test.size()); //capacity = 4, size = 0
    for(unsigned int i = 0; i < test.size(); i++)
    {
        printf("%dn", test.at(i));
    }
    return 0;
}

void reserve (size_type n);

调用容器大小，至少足够容纳n个元素。

如果n大于当前vector容器的容量，该函数将导致容器重新分配存储空间，将容量增加到n(或更大)。在所有其他情况下，函数调用不会导致重新分配，vector容量也不会受到影响。

#include 
#include 
#include 

int main()
{
    std::vector test;
    test.push_back(1);
    test.push_back(11);
    test.push_back(12);
    test.push_back(13);

    test.reserve(2); //
    
    printf("test.capacity = %lu, size = %lun", test.capacity(), test.size()); //capacity = 4, size = 0
    for(unsigned int i = 0; i < test.size(); i++)
    {
        printf("%dn", test.at(i));
    }
    return 0;
}

#include 
#include 
#include 

int main()
{
    std::vector test;
    test.push_back(1);
    test.push_back(11);
    test.push_back(12);
    test.push_back(13);

    test.reserve(12); //
    
    printf("test.capacity = %lu, size = %lun", test.capacity(), test.size()); //capacity = 4, size = 0
    for(unsigned int i = 0; i < test.size(); i++)
    {
        printf("%dn", test.at(i));
    }
    return 0;
}

 容量扩大，元素不变。