24 solver, with no access to binary_function or function<...>, thanks to an outdated compiler. :) Still using C++11 though.

24.cpp

#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
	
#define forn(i, n) for(int i = 0; i < (n); ++i)
#define FUNC(i) (*(dispatch[funcs[i]]))
#define MFUNC(o) ([](double a, double b) { return a o b; })
using namespace std;

typedef double (*oper) (double, double);
vector<oper> dispatch;
char ops[4] = {'+', '-', '*', '/'};

double t1(vector<double> num, vector<int> funcs) {
  return FUNC(0)(num[0], FUNC(1)(num[1], FUNC(2)(num[2], num[3])));
}

double t1m(vector<double> num, vector<int> funcs) {
  return FUNC(2)(FUNC(1)(FUNC(0)(num[0], num[1]), num[2]), num[3]);
}

double t2(vector<double> num, vector<int> funcs) {
  return FUNC(0)(num[0], FUNC(2)(FUNC(1)(num[1], num[2]), num[3]));
}

double t2m(vector<double> num, vector<int> funcs) {
  return FUNC(2)(FUNC(0)(num[0], FUNC(1)(num[1], num[2])), num[3]);
}

double t3(vector<double> num, vector<int> funcs) {
  return FUNC(1)(FUNC(0)(num[0], num[1]), FUNC(1)(num[2], num[3]));
}

int main() {
  dispatch.push_back(MFUNC(+));
  dispatch.push_back(MFUNC(-));
  dispatch.push_back(MFUNC(*));
  dispatch.push_back(MFUNC(/));

  double epsilon = 1e-5;
  vector<double> ns(4);
  forn(i, 4) cin >> ns[i];
  sort(ns.begin(), ns.end());
  do {
    forn(i, 4) forn(j, 4) forn(k, 4) {
      vector<int> fs(3);
      fs[0] = i;
      fs[1] = j;
      fs[2] = k;
      if(fabs(t1(ns, fs) - 24.0) < epsilon) {
        cout << ns[0] << " " << ops[fs[0]] << " ( " << ns[1] << " " << ops[fs[1]] << " ( " << ns[2] << " " << ops[fs[2]] << " " << ns[3] << " ) )" << endl;
      }

      if(fabs(t1m(ns, fs) - 24.0) < epsilon) {
        cout << "( ( " << ns[0] << " " << ops[fs[0]] << " " << ns[1] << " ) " << ops[fs[1]] << " " << ns[2] << " ) " << ops[fs[2]] << " " << ns[3] << endl;
      }

      if(fabs(t2(ns, fs) - 24.0) < epsilon) {
        cout << ns[0] << " " << ops[fs[0]] << " ( ( " << ns[1] << " " << ops[fs[1]] << " " << ns[2] << " ) " << ops[fs[2]] << " " << ns[3] << " ) " << endl; 
      }

      if(fabs(t2m(ns, fs) - 24.0) < epsilon) {
        cout << "( " << ns[0] << " " << ops[fs[0]] << " ( " << ns[1] << " " << ops[fs[1]] << " " << ns[2] << " ) ) " << ops[fs[2]] << " " << ns[3] << endl;  
      }

      if(fabs(t3(ns, fs) - 24.0) < epsilon) {
        cout << "( " << ns[0] << " " << ops[fs[0]] << " " << ns[1] << " ) " << ops[fs[1]] << " ( " << ns[2] << " " << ops[fs[2]] << " " << ns[3] << " ) " << endl;
      }
    } 
  } while(next_permutation(ns.begin(), ns.end()));
}