Question

I am having problem with string concatination.

std::deque<std::string> fileNamesVector;
double * res_array;
string *strarr;

size = fileNamesVector.size();
res_array = new double[size];
strarr = new string [size];

I need to append res_array with 4 spaces followed by filenamevector.
How can I do this.

strarr[i] = res_array[i] + "     " + fileNamesVector[i];

But it gives errors. Saying “exp must have arithmetic or enum type”
Please help.

Answer 1

In C++, there is no implicit conversion between a double, a char * or std::string.

res_array[i] + " " is trying to add a double to char *, so the compiler tries an implicit conversion, but none exists, so it gives you an error saying that operator+ needs numerical types.

You will instead need to explicitly convert res_array[i] to a string.

// File: convert.h
#include <iostream>
#include <sstream>
#include <string>
#include <stdexcept>

class BadConversion : public std::runtime_error {
public:
  BadConversion(std::string const& s)
    : std::runtime_error(s)
    { }
};

inline std::string stringify(double x)
{
  std::ostringstream o;
  if (!(o << x))
    throw BadConversion("stringify(double)");
  return o.str();
}

The example above is from The C++ FAQ, even though there are many stackoverflow questions dedicated to this topic, TC++FAQ deserves the real shout out for being OG 🙂

Or with C++11, use std::to_string

strarr[i] = std::to_string(res_array[i]) + "     " + fileNamesVector[i];