C++“std::string 尚未声明”错误
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C++ "std::string has not been declared" error
提问by wannabe programmer
I've been looking for an answer on websites but couldn't find any answer that helped me.
我一直在网站上寻找答案，但找不到任何对我有帮助的答案。
I have a code that uses strings when i tried (like suggested) to add these lines:
当我尝试（如建议的那样）添加这些行时，我有一个使用字符串的代码：
using namespace std;
using std::string;
#include <string>
I tried to use each of them seperately and I tried all of them together. The best situation was when all of the string errors disappeared but I had another weird error on the line "using std::string" and the error was: std::string has not been declared. Any ideas? Thanks guys.
我尝试单独使用它们中的每一个，然后将它们全部一起尝试。最好的情况是所有字符串错误都消失了，但我在“使用 std::string”行上遇到了另一个奇怪的错误，错误是：std::string 尚未声明。有任何想法吗？谢谢你们。
采纳答案by Bathsheba
Put #include <string>first.
#include <string>先放。
Avoid usingstatements in headers as you potentially bring in all sorts of stuff into many compilation units. using std::stringis perhaps acceptable in a header but using namespace stdcertainly isn'tas it will cause so much namespace pollution in all compilation units. The std namespace is continuously expanding (look at all the new stuff in C++), so you don't want to have to fix lots of errors when upgrading your compiler.
避免using在头文件中使用语句，因为您可能会将各种内容引入许多编译单元。using std::string在头文件中可能是可以接受的，但using namespace std肯定不是，因为它会在所有编译单元中造成如此多的命名空间污染。std 命名空间不断扩展（查看 C++ 中的所有新内容），因此您不希望在升级编译器时修复大量错误。
回答by Roee Gavirel
The includeshould come before the using
将include要来的前using
#include <string>
using namespace std;
//using std::string; <-- Needless
