C++ 将字符串传递给 file.open();
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Passing a string to file.open();
提问by Muricula
I am used to higher level languages (java, python etc.), where this is dead obvious. I am trying to pass a string the user inputs to cin, the name of a file to open. There appears to be some sort of pointer madness error, and my code will not compile. I deleted some of my code to make it more clear.
我习惯了高级语言（java、python 等），这很明显。我试图将用户输入的字符串传递给要打开的文件名 cin。似乎存在某种指针疯狂错误，我的代码无法编译。我删除了一些代码以使其更清晰。
#include <iostream>
#include <fstream>
using namespace std;
string hash(string filename);
int main(){
cout << "Please input a file name to hash\n";
string filename;
cin >> filename;
cout <<hash(filename);
return 0;
}
string hash(string filename){
file.open(filename);
if(file.is_open()){
file.close();
}
return returnval;
}
Here is the compile time error.
这是编译时错误。
<code>
$ g++ md5.cpp
md5.cpp: In function ‘std::string hash(std::string)':
md5.cpp:22: error: no matching function for call to ‘std::basic_ifstream<char, std::char_traits<char> >::open(std::string&)'
/usr/include/c++/4.2.1/fstream:518: note: candidates are: void std::basic_ifstream<_CharT, _Traits>::open(const char*, std::_Ios_Openmode) [with _CharT = char, _Traits = std::char_traits<char>]
</code>
(I know that there are libraries for md5 hashes, but I am trying to learn about how the hash works, and eventually hash collision)
（我知道有用于 md5 散列的库，但我正在尝试了解散列的工作原理，并最终了解散列冲突）
回答by chris
open()takes a C-style string. Use std::string::c_str()to get this:
open()采用 C 风格的字符串。使用std::string::c_str()得到这个：
file.open (filename.c_str());
In order to use just a string, as pointed out below, you'll need to use a compiler with C++11 support, as the overload was added for C++11.
为了仅使用字符串，如下所述，您需要使用支持 C++11 的编译器，因为 C++11 添加了重载。
The reason it's not like Java etc. is that it came from C. Classes didn't exist in C (well, not nearly as well as they do in C++), let alone a Stringclass. In order for C++ to provide a string class and keep compatibility, they need to be different things, and the class provides a conversion constructor for const char * -> std::string, as well as c_str()to go the other way.
它不像 Java 等的原因是它来自 C。C 中不存在类（嗯，不像在 C++ 中那样好），更不用说String类了。为了让 C++ 提供一个字符串类并保持兼容性，它们需要是不同的东西，并且该类为 提供了一个转换构造函数const char * -> std::string，以及c_str()走另一条路。
Consider passing the argument (and maybe the return too) as const std::string &as well; no unnecessary copies. The optimization would probably catch those, but it's always good to do.
也考虑传递参数（也可能是返回值）const std::string &；没有不必要的副本。优化可能会捕捉到这些，但这样做总是好的。
