假设我有std::vector<std::pair<int,Direction>>.
我正在尝试使用 erase-remove_if 习惯用法从向量中删除对。
stopPoints.erase(std::remove_if(stopPoints.begin(),
stopPoints.end(),
[&](const stopPointPair stopPoint)-> bool { return stopPoint.first == 4; }));
我想删除所有将 .first 值设置为 4 的对。
在我的示例中，我有对：
- 4, Up
- 4, Down
- 2, Up
- 6, Up
但是，在我执行erase-remove_if 之后，我只剩下：
- 2, Up
- 6, Up
- 6, Up
我在这里做错了什么？
正确的代码是：
stopPoints.erase(std::remove_if(stopPoints.begin(),
stopPoints.end(),
[&](const stopPointPair stopPoint)-> bool
{ return stopPoint.first == 4; }),
stopPoints.end());
您需要删除从返回的迭代器开始std::remove_if到向量末尾的范围，而不仅仅是单个元素。
“为什么？”
std::remove_if交换向量内的元素，以便将所有与谓词不匹配的元素放在容器的开头。这意味着如果谓词（lambda 函数的主体）返回true，则该元素将放置在向量的末尾。
remove_if然后**返回一个迭代器，它指向与谓词**匹配的第一个元素。换句话说，指向要删除的第一个元素的迭代器。
std::vector::erase 擦除从返回的迭代器到向量末尾的范围，以便删除与谓词匹配的所有元素。
The method std::vector::erase has two overloads:
iterator erase( const_iterator pos );
iterator erase( const_iterator first, const_iterator last );
The first one only remove the element at pos while the second one remove the range [first, last).
Since you forget the last iterator in your call, the first version is chosen by overload resolution, and you only remove the first pair shifted to the end by std::remove_if. You need to do this:
stopPoints.erase(std::remove_if(stopPoints.begin(),
stopPoints.end(),
[&](const stopPointPair stopPoint)-> bool {
return stopPoint.first == 4;
}),
stopPoints.end());
The erase-remove idiom works as follow. Let say you have a vector {2, 4, 3, 6, 4} and you want to remove the 4:
std::vector<int> vec{2, 4, 3, 6, 4};
auto it = std::remove(vec.begin(), vec.end(), 4);
Will transform the vector into {2, 3, 6, A, B} by putting the "removed" values at the end (the values A and B at the end are unspecified (as if the value were moved), which is why you got 6 in your example) and return an iterator to A (the first of the "removed" value).
If you do:
vec.erase(it)
...the first overload of std::vector::erase is chosen and you only remove the value at it, which is the A and get {2, 3, 6, B}.
By adding the second argument:
vec.erase(it, vec.end())
...the second overload is chosen, and you erase value between it and vec.end(), so both A and B are erased.