mirror of
https://github.com/Light-City/CPlusPlusThings.git
synced 2026-10-28 19:04:45 +08:00
dfe1c933b3
- const README: rewrite the space-saving claim per issue #5 follow-ups (constant-expression const vs #define immediates; which const qualify as constant expressions) - const README: fix the class-section diagnosis per issue #338 (overload resolution picks the const overload; the restriction only surfaces once that overload is removed) - sizeof README: virtual-inheritance result is compiler-dependent (GCC merges vbptr+vfptr -> 16, MSVC keeps them separate -> 32) #246 - static README: new section on internal linkage / access restriction with anonymous-namespace comparison #142 - 10-day practice: full problem list table + fixed day links #136 - new design_pattern/singleton/singleton_default.hpp: reusable thread-safe CRTP singleton template, compile+concurrency tested #303
45 lines
2.0 KiB
Markdown
45 lines
2.0 KiB
Markdown
> 来自 [C++那些事网站](https://light-city.github.io/stories_things/cpp-things/vector-resize-reverse/) 的文章，由站点同步脚本导出。
|
||
|
||
# C++ vector resize 与 reverse
|
||
|
||
在C++中，`resize`和`reserve`是用于容器（例如`std::vector`）的两个成员函数，用于管理vector的大小和内存分配。
|
||
|
||
例如：
|
||
|
||
```
|
||
std::vector<int> v1;
|
||
v1.resize(1000); //allocation + instance creation
|
||
cout <<(v1.size() == 1000)<< endl; //prints 1
|
||
cout <<(v1.capacity()==1000)<< endl; //prints 1
|
||
|
||
std::vector<int> v2;
|
||
v2.reserve(1000); //only allocation
|
||
cout <<(v2.size() == 1000)<< endl; //prints 0
|
||
cout <<(v2.capacity()==1000)<< endl; //prints 1
|
||
```
|
||
|
||
`resize`函数用于更改容器的大小，可以增加或减少容器中元素的数量。当增加容器的大小时，新添加的元素将会进行初始化。当减少容器的大小时，超出新大小的元素将会被移除。
|
||
|
||
`reserve`函数用于预分配容器的内存空间，但不改变容器的大小。它可以用于提前分配足够的内存，以避免频繁的重新分配内存操作，从而提高性能。但是，调用`reserve`并不会改变容器的大小，只是为容器分配足够的内存空间。
|
||
|
||
需要注意的是，`resize`和`reserve`函数在容器类型上的可用性有所不同。`resize`函数对于大多数容器类型是可用的，而`reserve`函数只对具有动态大小的容器类型有效，例如`std::vector`和`std::deque`。对于固定大小的容器类型（例如`std::array`），调用`reserve`没有任何效果。
|
||
|
||
从源码层面上来看：
|
||
|
||
```
|
||
void resize(size_type new_size, value_type value = value_type()) {
|
||
if (new_size < size()) {
|
||
erase(begin() + new_size, end()); // 移除超出新大小的元素
|
||
} else {
|
||
insert(end(), new_size - size(), value); // 添加新元素
|
||
}
|
||
}
|
||
void reserve(size_type _n) {
|
||
if (__n > max_size())
|
||
__throw_length_error(__N("vector::reserve"));
|
||
if (capacity() < __n)
|
||
_M_reallocate(__n);
|
||
}
|
||
```
|
||
|
||
> 本节完 |