Question

This is a portion of my simple program

string appData = getenv("APPDATA");
const char *mypath= (appData+"\\MyApplication\\hello.txt").c_str();      
cout << mypath;  
// output: c:\users\xrobot\appdata\Roaming\Myapplication\hello.txt   
fstream file(mypath,ios::in);
ofstream filetemp;    
filetemp.open("world.bak");
cout << mypath;  
// output: É↕7

Why is mypath changed in that weird string ?

Answer 1

You should use std::string as:

std::string appData = getenv("APPDATA");
std::string path = appData+"\\MyApplication\\hello.txt";

then do this:

const char * mypath = path.c_str();

Note that you must not do this:

const char* mypath = (appData+"\\MyApplication\\hello.txt").c_str();

It is because expression on the right hand side is a temporary which gets destroyed at the end of the expression and mypath will continue to point to the destroyed object. It becomes a dangling pointer, in other words.

—

Why is mypath changed in that weird string ?

Because in your posted code, mypath is a dangling pointer, using which invokes undefined behavior.

This is how you should write the code:

std::string appData = getenv("APPDATA");
std::string mypath= appData+"\\MyApplication\\hello.txt";
cout << mypath;  
fstream file(mypath.c_str(),ios::in);