C++ std::transform() 和 toupper() ..为什么会失败？
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C++ std::transform() and toupper() ..why does this fail?
提问by sivabudh
I have 2 std::string. I just want to, given the input string:
我有 2 个 std::string。我只想，给定输入字符串：
- capitalize every letter
- assign the capitalized letter to the output string.
- 大写每个字母
- 将大写字母分配给输出字符串。
How come this works:
这是怎么工作的：
std::string s="hello";
std::string out;
std::transform(s.begin(), s.end(), std::back_inserter(out), std::toupper);
but this doesn't (results in a program crash)?
但这不会（导致程序崩溃）？
std::string s="hello";
std::string out;
std::transform(s.begin(), s.end(), out.begin(), std::toupper);
because this works (at least on the same string:
因为这有效（至少在同一个字符串上：
std::string s="hello";
std::string out;
std::transform(s.begin(), s.end(), s.begin(), std::toupper);
回答by hrnt
There is no space in out. C++ algorithms do not grow their target containers automatically. You must either make the space yourself, or use a inserter adaptor.
中没有空间out。C++ 算法不会自动增长其目标容器。您必须自己制作空间，或使用插入器适配器。
To make space in out, do this:
要在 中腾出空间out，请执行以下操作：
out.resize(s.length());
out.resize(s.length());
[edit] Another option is to create the output string with correct size with this constructor.
[编辑] 另一种选择是使用此构造函数创建具有正确大小的输出字符串。
std::string out(s.length(), 'X');
std::string out(s.length(), 'X');
回答by Michael Krelin - hacker
I'd say that the iterator returned by out.begin()is not valid after a couple of increments for the empty string. After the first ++it's ==out.end(), then the behavior after the next increment is undefined.
我会说返回的迭代器out.begin()在空字符串的几个增量之后无效。在第一个++it's==out.end()之后，下一个增量之后的行为是未定义的。
After all this exactly what insert iterator is for.
毕竟这正是插入迭代器的用途。
回答by RED SOFT ADAIR
Thats the sense of a backinserter: It insertselements to a container. using begin(), you pass a iterator to a empty container and modify invalid iterators.
这就是 backinserter 的意义：它将元素插入到容器中。使用 begin()，您将迭代器传递给空容器并修改无效的迭代器。
I am sorry - my edits interfered with your comments. I first posted something wrong accidentally.
很抱歉 - 我的编辑干扰了您的评论。我第一次不小心发布了一些错误的东西。
