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I'm new to C++ and working on a simple project. Basically where I'm encountering a problem is creating a file with a number (int) in the file name. As I see it,I have to first convert the int to a string (or char array) and then concatenate this new string with the rest of the file name.

Here is my code so far that fails to compile:

int n; //int to include in filename
char buffer [33];
itoa(n, buffer, 10);
string nStr = string(buffer);

ofstream resultsFile;
resultsFile.open(string("File - ") + nStr + string(".txt"));

This gives a couple compilation errors (compiling in Linux):

  1. itoa not declared in this scope
  2. no matching function for call to ‘std::basic_ofstream char, std::char_traits char ::open(std::basic_string char, std::char_traits char , std::allocator char )’

I've tried the advice here: c string and int concatenation and here: Easiest way to convert int to string in C++ with no luck.

If I using the to_string method, I end up with the error "to_string not a member of std".

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Lukas Bystricky
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  • If you can afford C++11, a much easier way to convert an integer to a string is to use `std::to_string`. See my answer for an example. – vitaut Sep 10 '13 at 19:11

6 Answers6

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You could use a stringstream to construct the filename.

std::ostringstream filename;
filename << "File - " << n << ".txt";
resultsFile.open(filename.str().c_str());
cdmh
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  • This looks promising. However when I compile the code I get the error: "aggregate ‘std::ostringstream filename’ has incomplete type and cannot be defined" – Lukas Bystricky Sep 10 '13 at 18:57
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    I don't recognise that error. You need to `#include ` at the top of your source file. – cdmh Sep 10 '13 at 19:01
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You want to use boost::lexical_cast. You also need to include any needed headers:

#include <boost/lexical_cast>
#include <string>
std::string nStr = boost::lexical_cast<std::string>(n);

then it's simply:

std::string file_name = "File-" + nStr + ".txt";

because std::strng plays nicely with string literals (e.g. ".txt").

Paul Evans
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For itoa, you are likely missing #include <stdlib.h>. Note that itoa is non-standard: the standard ways to format an integer as string as sprintf and std::ostringstream.

ofstream.open() takes a const char*, not std::string. Use .c_str() method to obtain the former from the latter.

Putting it together, you are looking for something like this:

ostringstream nameStream;
nameStream << "File - " << n << ".txt";
ofstream resultsFile(nameStream.str().c_str());
Igor Tandetnik
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1

Using std::ostringstream:

std::ostringstream os;
os << "File - "  << nStr << ".txt";
std::ofstream resultsFile(os.str().c_str());

Using std::to_string (C++11):

std::string filename = "File - " + std::to_string(nStr) + ".txt";
std::ofstream resultsFile(filename.c_str());
vitaut
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0

for itoa function 

include <stdlib.h>

consider this link

http://www.cplusplus.com/reference/cstdlib/itoa/

user2766182
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0

You can use std::stringstream

std::stringstream ss;
ss << "File - " << n << ".txt";

Since the constructor requires a char pointer, you need to convert it into a char pointer using

ofstream resultsFile(ss.str().c_str());