c++: ifstream open problem with passing a string f

问题:

This question already has an answer here:

• No matching function - ifstream open() 1 answer

i'm trying to pass a string from main to another function. this string is a name of text file that needs to be oepened. as far as i can see, i am passing the string alright, but when i try to use ifstream.open(textFileName), it doesn't quite work. but when i manually hardcode it as ifstream.open("foo.txt"), it works just fine. i would need to use this function several times so i would like to be able to pass in a string of text file name..

here's my main

#ifndef DATA_H
#define DATA_H
#include "Data.h"
#endif

#ifndef DATAREADER_H
#define DATAREADER_H
#include "DataReader.h"
#endif

using namespace std;

int main()
{
 vector<Data*> database = DataReader("foo.txt");

 return 0; 
}

header of DataReader

#include <fstream>
#include <iostream>
#include <vector>
#include <string>

#ifndef DATA_H
#define DATA_H
#include "Data.h"
#endif

using namespace std;

vector<Data*> DataReader(string textFile);

and finally the DataReader.cpp

#include "DataReader.h"

using namespace std;

vector<Data*> DataReader(string textFile)
{
 ifstream aStream;     
 aStream.open(textFile); //line 11

i looked up the ifstream.open() and it takes a string and a mode as parameters. not really sure what to do with the modes, but i tried them but they gave the same error message

DataReader.cpp: In function 'std::vector<Data*, std::allocator<Data*> > DataReader(std::string)':
DataReader.cpp:11: error: no matching function for call to 'std::basic_ifstream<char, std::char_traits<char> >::open(std::string&)'
/usr/local/lib/gcc/sparc-sun-solaris2.9/4.0.3/../../../../include/c++/4.0.3/fstream:495: note: candidates are: void std::basic_ifstream<_CharT, _Traits>::open(const char*, std::_Ios_Openmode) [with _CharT = char, _Traits = std::char_traits<char>]

thank you in advance for any input/suggestions.

Dean

回答1:

the standard streams doesn't accept a standard string, only c-string! So pass the string using c_str():

aStream.open(textFile.c_str());

回答2:

Try this:

aStream.open(textFile.c_str()); //line 11

I think your code needs to take the internal C string to pass to the open() call. Note I'm not at a compiler right now, so can't double check this.

You may also want to check the signature of the this method:

vector<Data*> DataReader(string textFile);

Here, a complete copy of the vector will be taken when it's returned from the method, which could be computationally expensive. Note, it won't copy the Data objects, just the pointers, but with a lot of data might not be a good idea. Similarly with the string input.

Consider this instead:

void DataReader( const string& textFile, vector<Data*>& dataOut );

回答3:

ifstream open takes a const char* pointer as parameter, use c_str() function of std::string to get this pointer. You can see the meaning of the parameters here