C++ lambda with captures as a function pointer

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I just ran into this problem.

The code compiles fine without lambda captures, but there is a type conversion error with lambda capture.

Solution with C++11 is to use std::function (edit: another solution that doesn’t require modifying the function signature is shown after this example). You can also use boost::function (which actually runs significantly faster). Example code – changed so that it would compile, compiled with gcc 4.7.1:

#include <iostream>
#include <vector>
#include <functional>

using namespace std;

int ftw(const char *fpath, std::function<int (const char *path)> callback) {
  return callback(fpath);
}

int main()
{
  vector<string> entries;

  std::function<int (const char *fpath)> callback = [&](const char *fpath) -> int {
    entries.push_back(fpath);
    return 0;
  };

  int ret = ftw("/etc", callback);

  for (auto entry : entries ) {
    cout << entry << endl;
  }

  return ret;
}

Edit:
I had to revisit this when I ran into legacy code where I couldn’t modify the original function signature, but still needed to use lambdas. A solution that doesn’t require modifying the function signature of the original function is below:

#include <iostream>
#include <vector>
#include <functional>

using namespace std;

// Original ftw function taking raw function pointer that cannot be modified
int ftw(const char *fpath, int(*callback)(const char *path)) {
  return callback(fpath);
}

static std::function<int(const char*path)> ftw_callback_function;

static int ftw_callback_helper(const char *path) {
  return ftw_callback_function(path);
}

// ftw overload accepting lambda function
static int ftw(const char *fpath, std::function<int(const char *path)> callback) {
  ftw_callback_function = callback;
  return ftw(fpath, ftw_callback_helper);
}

int main() {
  vector<string> entries;

  std::function<int (const char *fpath)> callback = [&](const char *fpath) -> int {
    entries.push_back(fpath);
    return 0;
  };
  int ret = ftw("/etc", callback);

  for (auto entry : entries ) {
    cout << entry << endl;
  }

  return ret;
}

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