C++ Concating strings result in "invalid operands of types ‘const char*’ and ‘const char"

Question

I want to concat two strings but i get error that i don't understand how to overcome this error.

Is there some way to convert this const char* to char? Should i use some dereferencing?

../src/main.cpp:38: error: invalid operands of types ‘const char*’ and ‘const char [2]’ to binary ‘operator+’
make: *** [src/main.o] Error 1

However, if i try to make up "bottom" string this way, it works:

bottom += "| ";
bottom += tmp[j];
bottom += " ";

Here is the code.

#include <iostream>
#include <fstream>
#include <vector>
#include <string>
#include <iterator>
#include <sstream>

int main(int argc, char* argv[]) {

    ifstream file("file.txt");

    vector<string> mapa;
    string line, top, bottom;

    while(getline(file,line)){
        mapa.push_back(line);
    }

    string tmp;
    for(int i = 0; i < mapa.size(); i++)
    {
        tmp = mapa[i];
        for(int j = 0; j < tmp.size(); j++)
        {
            if(tmp[j] != ' ')
            {
                top += "+---";
                bottom += "| " + tmp[j] + " ";
            } else {

            }
        }
        cout << top << endl;
        cout << bottom << endl;
    }

    return 0;
}

Answer 1

Here:

bottom += "| " + tmp[j] " ";

You are trying to sum a char and a pointer to char. That won't work (it won't result in the concatenation of the character and the pointed string literal). And the same is true if you add a + sign after tmp[j], because that will still be evaluated as (extra parentheses added to stress the fact that operator + associates to the left):

bottom += ("| " + tmp[j]) + " "; // ERROR!
//         ^^^^^^^^^^^^^
//         This is still summing a character and a pointer,
//         and the result will be added to another pointer,
//         which is illegal.

If you want to have everything in one line, just do:

bottom += std::string("| ") + tmp[j] + " ";

Now, the above expression on the right side of the assignment will be evaluated as:

(std::string("| ") + tmp[j]) + " ";

Since operator + for a std::string and a char is defined and returns an std::string, the result of evaluating the sub-expression within parentheses will be an std::string, which is then summed to the string literal " ", returning (again) an std::string.

Eventually, the result of the whole expression (std::string("| ") + tmp[j]) + " " is given in input to operator +=.

Answer 2

Looks to be that your problem is on this line:

bottom += "| " + tmp[j] " ";

You're missing a + between tmp[j] and " ". Try changing it to:

bottom += "| " + tmp[j] + " ";

Edit:

The above will still cause a compilation error, and the one you stated w/ g++, the following will work and result in minimal temporary objects:

bottom += std::string("| ") + tmp[j] + " ";