Question

void searchString(const string selection,const string filename)
{
    ifstream myfile;
    string sline;
    string sdata;
    myfile.open(filename);
    while(!myfile.eof())
    {
        getline(myfile,sline);
        sdata = sdata + sline;
    }

How do i use string filename as myfile.open(filename)

Initially i was using file.txt instead, but if i use a variable that pass in by the function, like string filename, it give me an error

myfile.open("file.txt");

Error message is as followed:

main.cpp:203:25: error: no matching function for call to ‘std::basic_ifstream<char>::open(const string&)’
main.cpp:203:25: note: candidate is:
/usr/include/c++/4.6/fstream:531:7: note: void std::basic_ifstream<_CharT, _Traits>::open(const char*, std::ios_base::openmode) [with _CharT = char, _Traits = std::char_traits<char>, std::ios_base::openmode = std::_Ios_Openmode]
/usr/include/c++/4.6/fstream:531:7: note:   no known conversion for argument 1 from ‘const string {aka const std::basic_string<char>}’ to ‘const char*’
make: *** [main.o] Error 1

Answer 1

The constructor for std::ifstream::open (for the particular standard of C++ that you’re using) doesn’t allow a std::string argument, so you have to use:

 myfile.open(filename.c_str());

The constructor expects type const char * which you can obtain from a std::string object using its c_str() member function.