Assign a string from an iterator - c++

Question

Here is a problem I have , I have a vector of filenames and I want to check if they end by .jpg or by .png so I made some code with iterators and the STL, this i also for creating a std::map with those names as keys and with value a texture, so here is my code, that does a Segmentation Fault error at the line 11:
#include "TextureManager.h"
std::map<std::string,sf::Texture> createTextureArray(){
std::map<std::string,sf::Texture> textureArray;
std::vector<std::string>::iterator strIt;
std::string fName;
for(strIt = textureFileList().begin(); strIt != textureFileList().end(); strIt++){
sf::Texture texture;
fName = *strIt;
if(fName[fName.size()] == 'g' && fName[fName.size()-1] == 'p' && fName[fName.size()-2] == 'j' && fName[fName.size()-3] == '.'){
texture.loadFromFile(fName);
textureArray[fName] = texture;
}
else if(fName[fName.size()] == 'g' && fName[fName.size()-1] == 'n' && fName[fName.size()-2] == 'p' && fName[fName.size()-3] == '.'){
texture.loadFromFile(fName);
textureArray[fName] = texture;
}
}
return textureArray;
}
I think this is the only code needed to try to understand the problem , but if anyone wants more of this code here is the Github repo

Answer 1

This is not shown in your question, but textureFileList returns by value, which means that you get a copy of the std::vector<std::string> it returns. You're calling this function twice, once for begin() and then once for end(), which means you're calling those functions on different copies of the vector. Obviously the beginning of one copy has no relation to the end of another copy. Not only this, but those copies are being destroyed immediately afterwards, because they are temporaries. Instead, you should store a single copy and call begin and end on that:
std::vector<std::string> fileList = textureFileList();
for(strIt = fileList.begin(); strIt != fileList.end(); strIt++){

Answer 2

And fName[fName.size()] is always '\0'.
You should use
if(fName[fName.size()-1] == 'g' && fName[fName.size()-2] == 'p' && fName[fName.size()-3] == 'j' && fName[fName.size()-4] == '.'){
and
else if(fName[fName.size()-1] == 'g' && fName[fName.size()-2] == 'n' && fName[fName.size()-3] == 'p' && fName[fName.size()-4] == '.'){

Answer 3

I suppose that function textureFileList() returns an object of type std::vector<std::string> by reference.
The segmentation fault can occur due to accessing non-existent character of a string.
if(fName[fName.size()] == 'g' && fName[fName.size()-1] == 'p' && fName[fName.size()-2] == 'j' && fName[fName.size()-3] == '.'){
There is no such character of the string with index fName.size(). The valid range of indexes is 0, size() -1 provided that size() is not equal to zero.
You should use another approach. You have to find the period using member function rfind() and then compare the substring with a given extension.
Here is an example
#include <iostream>
#include <string>
int main()
{
std::string s( "SomeFile.jpg" );
std::string::size_type n = s.rfind( '.' );
if ( n != std::string::npos && s.substr( n ) == ".jpg" )
{
std::cout << "It is a JPEG file" << std::endl;
}
return 0;
}
The output is
It is a JPEG file
Your for loop could be written the following way
for ( const std::string &fName : textureFileList() )
{
const char *ext[] = { "jpg", "png" };
std::string::size_type n = s.rfind( '.' );
if ( n != std::string::npos &&
( fName.substr( n + 1 ) == ext[0] || fName.substr( n + 1 ) == ext[1] ) )
{
sf::Texture texture;
texture.loadFromFile( fName );
textureArray[fName] = texture;
}
}
If you need to erase the extension then you can write simply
fName.erase( n );
In this case fName has to be defined as a non-const reference in the for statement.