c++ read and write files simultaneously

Question

I am trying to read all .png files in a folder then get their gray scale and rewrite them in a new folder (with the same name).

my path_in files : 1.png, 2.png, ... (different number of total files everytime)

so by the end, path_out should contains same numbers of png files, all with the same name as in path_in.

But I am now stuck. As I am new to c++, I could not get myself to get this right.
The error is on the overloads '+' operators.

int main()
{
Mat pic,gray;
vector<cv::String> fn;
glob(path_in + "*.png", fn, false);

vector<Mat> images;
size_t count = fn.size(); 
for (size_t i = 1; i < count + 1; i++)
{
    pic = imread(path_in + i, CV_LOAD_IMAGE_COLOR);
    cvtColor(pic, gray, CV_BGR2GRAY);

    imwrite(path_out + i + ".png", gray); 

}
}

Edited code:

#include "stdafx.h"
#include "iostream"
#include <direct.h>
#include "opencv2/core/core.hpp"
#include "opencv2/highgui/highgui.hpp"
#include "opencv2/imgproc/imgproc.hpp"

using namespace cv;
using namespace std;

std::string path_in("C:/Users/t/");
std::string path_out("C:/Users/New folder/");

int main()
{
    _mkdir(path_out.c_str()); //create folder if does not exist

    Mat pic, gray;

    vector<cv::String> fn;
    glob(path_in + "*.png", fn, false);

    //vector<Mat> images;
    size_t count = fn.size();
    for (size_t i = 1; i < count; ++i)
    {
        pic = imread(path_in + fn[i], CV_LOAD_IMAGE_COLOR);
        cvtColor(pic, gray, CV_BGR2GRAY); //error : Unhandled exception

        imwrite(path_out + fn[i] + ".png", gray);
    }

waitKey(0);
return 0;
}  

**apologize for not showing the complete code at the beginning, here is the combination code of mine with the suggested answer.
can anyone shed some lights?

Answer 1

Assuming path_in and path_out are std::string or cv::String variables, you can't use the + operator to append size_t values as-is to them, you have to convert the integer values to std::string/cv::String values first, eg:

int main()
{
    Mat pic, gray;
    vector<cv::String> fn;
    glob(path_in + "*.png", fn, false);

    size_t count = fn.size();
    for (size_t i = 1; i <= count; ++i)
    {
        pic = imread(path_in + std::to_string(i) + ".png", CV_LOAD_IMAGE_COLOR);
        cvtColor(pic, gray, CV_BGR2GRAY);

        imwrite(path_out + std::to_string(i) + ".png", gray);
    }
    return 0;
}

However, you shouldn't assume the filenames are sequentially numbered. glob() returns a vector of strings, so presumably it returns the actual filenames, so use them as-is instead, eg:

int main()
{
    Mat pic, gray;
    vector<cv::String> fn;
    glob(path_in + "*.png", fn, false);

    size_t count = fn.size();
    for (size_t i = 0; i < count; ++i)
    {
        pic = imread(path_in + fn[i], CV_LOAD_IMAGE_COLOR);
        cvtColor(pic, gray, CV_BGR2GRAY);

        imwrite(path_out + fn[i], gray);
    }
    return 0;
}