Cannot create a raster stack, because I cannot concatenate strings (dir_name + filename)

I am playing around with C++ and GDAL, making some basic exercises. And one of the things I wanted to build is to create a program that reads from a directory name the files within it, then in a foor loop it reads the dataset and writes out a the band into a new virtual raster. I managed to read from the directory the files within using the boost library. However when I wanted to create the raster it said that could not find the file name. I wanted to go around the problem by creating a new variable called bdir, and then concatenate the file's name, so I could pass it to the GDALOpenShared function.

Here is my code so you might have a better idea What I would like to accomplish:

#include <string>
#include <iostream>
#include <boost/filesystem.hpp>
#include <boost/algorithm/string/predicate.hpp>
#include "gdal/gdal.h"
#include "gdal/cpl_conv.h" /* for CPLMalloc() */
#include "gdal/vrtdataset.h"
#include <iostream>
#include <string>
#include "gdal/cpl_string.h"
#include <stdbool.h>


using namespace std;
using namespace boost::filesystem;

int main()
{
    GDALAllRegister();

    path p("/home/roger/Documents/09_Stack_Raster/media");
    GDALDriver *poDriver = (GDALDriver*) GDALGetDriverByName( "VRT" );
    GDALDataset *poSrcDS, *poVRTDS;
    string bdir = "media/";

    for (auto i = directory_iterator(p); i != directory_iterator(); i++)
    {   
        string filename = i -> path().filename().string();
        cout << filename << endl;
        if (!is_directory(i->path()) && boost::ends_with(filename, ".tif"))
        {
            cout << i->path().filename().string() << endl;
            std::string str = "media/" + i->path().filename().string().c_str();
            poSrcDS = (GDALDataset *) GDALOpenShared(i->path().filename().string().c_str(), GA_ReadOnly);
            poVRTDS = poDriver->CreateCopy("media/B02_stack.vrt", poSrcDS,FALSE,NULL,NULL,NULL);
            GDALClose((GDALDatasetH) poSrcDS);
        }

        else
            continue;
    }

    GDALClose((GDALDatasetH) poVRTDS);

}

I receive however the following error:

stack_raster.cpp: In function 'int main()':
stack_raster.cpp:33:40: error: invalid operands of types 'const char [7]' and 'const char*' to binary 'operator+'
std::string str = "media/" + i->path().filename().string().c_str();

Could someone help me here please? I checked this answer in Error: invalid operands of types ‘const char [35]’ and ‘const char [2]’ to binary ‘operator+’

and it is suggested that to be sure that string concatenation works in c++, both arguments should be string, which is something I did by declaring the bdir as string and adding the transformation from char to string by using .c_str() method in the path iterator.

Could someone give me some hints about it?

Solution

std::string str = "media/" + i->path().filename().string().c_str();

This does not compile because both of the operands to the + operator are const char* types [1] (sometimes known as "C strings"), not std::string types.

Remove the final .c_str() (which is unnecessary because you want to concatenate the std::string, not the C string version) from the second operand and it should work.

[1] Technically the first operand is a const char [7] type, which is not exactly the same as const char*, but for the purposes of this question what matters is that these types cannot be concatenated using the + operator.