C++ std::string 比较（检查字符串是否以另一个字符串开头）
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std::string comparison (check whether string begins with another string)
提问by Hymanhab
I need to check whether an std:string begins with "xyz". How do I do it without searching through the whole string or creating temporary strings with substr().
我需要检查 std:string 是否以“xyz”开头。如何在不搜索整个字符串或使用 substr() 创建临时字符串的情况下执行此操作。
回答by Wacek
I would use compare method:
我会使用比较方法：
std::string s("xyzblahblah");
std::string t("xyz")
if (s.compare(0, t.length(), t) == 0)
{
// ok
}
回答by Neutrino
An approach that might be more in keeping with the spirit of the Standard Library would be to define your own begins_with algorithm.
一种可能更符合标准库精神的方法是定义您自己的 begin_with 算法。
#include <algorithm>
using namespace std;
template<class TContainer>
bool begins_with(const TContainer& input, const TContainer& match)
{
return input.size() >= match.size()
&& equal(match.begin(), match.end(), input.begin());
}
This provides a simpler interface to client code and is compatible with most Standard Library containers.
这为客户端代码提供了更简单的接口，并与大多数标准库容器兼容。
回答by Alex Ott
Look to the Boost's String Algolibrary, that has a number of useful functions, such as starts_with, istart_with (case insensitive), etc. If you want to use only part of boost libraries in your project, then you can use bcp utility to copy only needed files
看看Boost的String Algo库，它有很多有用的函数，比如starts_with、istart_with（不区分大小写）等。如果你只想在你的项目中使用boost库的一部分，那么你可以使用bcp工具来复制只需要文件
回答by Alejadro Xalabarder
It seems that std::string::starts_with is inside C++20, meanwhile std::string::find can be used
似乎 std::string::starts_with 在 C++20 中，同时可以使用 std::string::find
std::string s1("xyzblahblah");
std::string s2("xyz")
if (s1.find(s2) == 0)
{
// ok, s1 starts with s2
}
回答by 1800 INFORMATION
I feel I'm not fully understanding your question. It looks as though it should be trivial:
我觉得我没有完全理解你的问题。看起来它应该是微不足道的：
s[0]=='x' && s[1]=='y' && s[2]=='z'
This only looks at (at most) the first three characters. The generalisation for a string which is unknown at compile time would require you to replace the above with a loop:
这仅查看（最多）前三个字符。在编译时未知的字符串的泛化需要您用循环替换上述内容：
// look for t at the start of s
for (int i=0; i<s.length(); i++)
{
if (s[i]!=t[i])
return false;
}
