在c++中迭代一个向量的正确方法是什么?
考虑这两个代码片段，这一个工作得很好:
for (unsigned i=0; i < polygon.size(); i++) {
sum += polygon[i];
}
还有这个:
for (int i=0; i < polygon.size(); i++) {
sum += polygon[i];
}
生成警告:有符号整数表达式和无符号整数表达式之间的比较。
对我来说，无符号变量看起来有点吓人，我知道无符号变量如果使用不当会很危险，所以-这是正确的吗?
当前回答
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) {
...
其他回答
为了完整，c++ 11语法只支持另一个版本的迭代器(ref):
for(auto it=std::begin(polygon); it!=std::end(polygon); ++it) {
// do something with *it
}
哪一种也适合反向迭代
for(auto it=std::end(polygon)-1; it!=std::begin(polygon)-1; --it) {
// do something with *it
}
模糊但重要的细节:如果你像下面这样说“for(auto it)”，你得到的是对象的副本，而不是实际的元素:
struct Xs{int i} x;
x.i = 0;
vector <Xs> v;
v.push_back(x);
for(auto it : v)
it.i = 1; // doesn't change the element v[0]
要修改vector的元素，需要将迭代器定义为引用:
for(auto &it : v)
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) {
...
我通常使用BOOST_FOREACH:
#include <boost/foreach.hpp>
BOOST_FOREACH( vector_type::value_type& value, v ) {
// do something with 'value'
}
它适用于STL容器、数组、c风格字符串等。
for (vector<int>::iterator it = polygon.begin(); it != polygon.end(); it++)
sum += *it;
