How to pass functions defined in a class as arguments to functions? - c++

Question

getMessage method extracts the first letter in each word of input string.
Example:
input = "Find the first letters of this Sentence"
output = FtflotS
#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <cctype>
using namespace std;
class HiddenMessage {
public:
bool space (char c) {
return isspace(c);
}
bool not_space (char c) {
return !isspace (c);
}
string getMessage(string text) {
string ret;
typedef string::const_iterator iter;
iter i, j;
i = text.begin();
while (i != text.end()) {
i = find_if (i, text.end(), not_space); // error here
j = find_if (i, text.end(), space); // error here
if (i != text.end()) {
ret += *i;
}
i = j;
}
return ret;
}
};
//compiler error:
//error: invalid use of non-static member function
I tried making definitions of space and not_space static and it did
not work.
getMessage is called from the main below:
#include <ctime>
#include <cmath>
#include <string>
#include <vector>
#include <sstream>
#include <iostream>
#include <algorithm>
using namespace std;
int main(int argc, char* argv[])
{
if (argc == 1)
{
cout << "Testing HiddenMessage (250.0 points)" << endl << endl;
for (int i = 0; i < 20; i++)
{
ostringstream s; s << argv[0] << " " << i;
int exitCode = system(s.str().c_str());
if (exitCode)
cout << "#" << i << ": Runtime Error" << endl;
}
int T = time(NULL)-1456061889;
double PT = T/60.0, TT = 75.0;
cout.setf(ios::fixed,ios::floatfield);
cout.precision(2);
cout << endl;
cout << "Time : " << T/60 << " minutes " << T%60 << " secs" << endl;
cout << "Score : " << 250.0*(.3+(.7*TT*TT)/(10.0*PT*PT+TT*TT)) << " points" << endl;
}
else
{
int _tc; istringstream(argv[1]) >> _tc;
HiddenMessage _obj;
string _expected, _received;
time_t _start = clock();
switch (_tc)
{
case 0:
{
string text = "compete online design event rating";
_expected = "coder";
_received = _obj.getMessage(text); break;
}
case 1:
{
string text = " c o d e r ";
_expected = "coder";
_received = _obj.getMessage(text); break;
}
case 2:
{
string text = "round elimination during onsite contest";
_expected = "redoc";
_received = _obj.getMessage(text); break;
}
case 3:
{
string text = " ";
_expected = "";
_received = _obj.getMessage(text); break;
}
/*case 4:
{
string text = ;
_expected = ;
_received = _obj.getMessage(text); break;
}*/
/*case 5:
{
string text = ;
_expected = ;
_received = _obj.getMessage(text); break;
}*/
/*case 6:
{
string text = ;
_expected = ;
_received = _obj.getMessage(text); break;
}*/
default: return 0;
}
cout.setf(ios::fixed,ios::floatfield);
cout.precision(2);
double _elapsed = (double)(clock()-_start)/CLOCKS_PER_SEC;
if (_received == _expected)
cout << "#" << _tc << ": Passed (" << _elapsed << " secs)" << endl;
else
{
cout << "#" << _tc << ": Failed (" << _elapsed << " secs)" << endl;
cout << " Expected: " << "\"" << _expected << "\"" << endl;
cout << " Received: " << "\"" << _received << "\"" << endl;
}
}
}

Answer 1

You have two problems.
The first, is you are supplying non-static class member functions (space and not_space) to find_if which expects a function object or pointer. So, declare them static if you want them to remain your class, or make them global by placing them outside the class.
The second, your string text parameter is non-const, but, you are working with a const interator type. begin() and end() calls will return const or non-const iterator depending on the calling object (text in this case) and whether or not it is const qualified. So, declare your text parameter as const.