File Write Thread safe C++ - c++

Question

I am getting error for the below code
I am implementing thread safe write to files as below ::
This is for only learning purpose, i am adding more text because stack overflow wants me to overflow the text.
class OpenFile
{
string fileName;
static map<string, unique_ptr<mutex>> fmap;
bool flag;
public :
OpenFile(string file) : fileName(file) {
try {
if(checkFile(file))
{
flag = false;
fmap.emplace(file, make_unique<mutex>());
}
else
{
flag = true;
}
}
catch(string str)
{
cout << str << endl;
}
}
void writeToFile(const string& str) const
{
if (flag)
{
lock_guard<mutex> lck(*fmap.find(fileName)->second);
ofstream ofile(fileName, ios::app);
ofile << "Writing to the file " << str << endl;
ofile.close();
}
else
{
ofstream ofile(fileName, ios::app);
ofile << "Writing to the file " << str << endl;
ofile.close();
}
}
string ReadFile() const
{
string line;
if (flag)
{
lock_guard<mutex> lck(*fmap.find(fileName)->second);
ifstream ifile(fileName, ios::in);
getline(ifile, line);
ifile.close();
}
else
{
ifstream ifile(fileName, ios::in);
getline(ifile, line);
ifile.close();
}
return line;
}
OpenFile() = delete;
OpenFile& operator=(const OpenFile& o) = delete;
static bool checkFile(string& fname);
};
bool OpenFile::checkFile(string& fname)
{
if (fmap.find(fname)==fmap.end())
{
return true;
}
else
return false;
}
Initial error was because of not passing data by ref to thread..
Error from VS2015 ::
Description Project Line Suppression State
'bool OpenFile::checkFile::<lambda_2bd4a02838a970867505463f4b7b6b9e>::operator ()(const std::pair<std::string,std::unique_ptr<std::mutex,std::default_delete<_Ty>>>) const': cannot convert argument 1 from 'std::pair<const _Kty,_Ty>' to 'const std::pair<std::string,std::unique_ptr<std::mutex,std::default_delete<_Ty>>>' BoQian 43
I think i have fixed it now, thank you

Answer 1

You dont have to use find_if at all in your checkFile function.
This is how you should do:
bool OpenFile::checkFile(string& fname)
{
return fmap.find(fname) != fmap.end();
}