关于向后迭代，请参阅这个答案。

Iterating forwards is almost identical. Just change the iterators / swap decrement by increment. You should prefer iterators. Some people tell you to use std::size_t as the index variable type. However, that is not portable. Always use the size_type typedef of the container (While you could get away with only a conversion in the forward iterating case, it could actually go wrong all the way in the backward iterating case when using std::size_t, in case std::size_t is wider than what is the typedef of size_type):

使用std::向量

使用迭代器

for(std::vector<T>::iterator it = v.begin(); it != v.end(); ++it) {
    /* std::cout << *it; ... */
}

重要的是，对于您不知道定义的迭代器，始终使用前缀增量形式。这将确保您的代码尽可能通用地运行。

使用Range c++ 11

for(auto const& value: a) {
     /* std::cout << value; ... */

使用索引

for(std::vector<int>::size_type i = 0; i != v.size(); i++) {
    /* std::cout << v[i]; ... */
}

使用数组

使用迭代器

for(element_type* it = a; it != (a + (sizeof a / sizeof *a)); it++) {
    /* std::cout << *it; ... */
}

使用Range c++ 11

for(auto const& value: a) {
     /* std::cout << value; ... */

使用索引

for(std::size_t i = 0; i != (sizeof a / sizeof *a); i++) {
    /* std::cout << a[i]; ... */
}

但是，请阅读向后迭代的答案，看看sizeof方法可以解决什么问题。

如果你的编译器支持它，你可以使用一个基于范围的for来访问vector元素:

vector<float> vertices{ 1.0, 2.0, 3.0 };

for(float vertex: vertices){
    std::cout << vertex << " ";
}

指纹:1 2 3。注意，您不能使用此技术来更改向量的元素。

for (vector<int>::iterator it = polygon.begin(); it != polygon.end(); it++)
    sum += *it; 

关于Johannes Schaub的回答:

for(std::vector<T*>::iterator it = v.begin(); it != v.end(); ++it) { 
...
}

这可能适用于某些编译器，但不适用于gcc。这里的问题是std::vector::iterator是类型、变量(成员)还是函数(方法)。使用gcc会得到以下错误:

In member function ‘void MyClass<T>::myMethod()’:
error: expected `;' before ‘it’
error: ‘it’ was not declared in this scope
In member function ‘void MyClass<T>::sort() [with T = MyClass]’:
instantiated from ‘void MyClass<T>::run() [with T = MyClass]’
instantiated from here
dependent-name ‘std::vector<T*,std::allocator<T*> >::iterator’ is parsed as a non-type, but instantiation yields a type
note: say ‘typename std::vector<T*,std::allocator<T*> >::iterator’ if a type is meant

解决方案是使用关键字'typename'，如下所示:

typename std::vector<T*>::iterator it = v.begin();
for( ; it != v.end(); ++it) {
...

auto polygonsize = polygon.size(), i=polygonsize;
for (i=0; i < polygonsize; i++) {
    sum += polygon[i];
}

This

uses auto to avoid us worrying about types. It takes any function calls e.g. the size() function call out of the loop to avoid unnecessary repeated function calls. It makes the loop counter available. Purists will want to work with the n'th element with no knowledge of the value of n, and see this as bad. It appears to have an unecessary statement i=polygonsize initializing the loop variable when it's declared, but this should disappear if there is a half decent code optimizer, and is merely to ensure i has the correct type.

我并不是说任何人都应该像我刚才那样编写代码。

我只是提供它作为另一种替代方案，它可以避免担心类型，将函数调用从循环中取出，并使循环计数器可用于更复杂场景中的调试信息等实际工作。

第一个是类型正确，严格意义上的正确。(如果你想的是，size永远不可能小于零。)不过，这个警告在我看来是一个很好的被忽视的警告。