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Lecture 12, Thu 11/07
Function Pointers
Function Pointers
- Consider the
library, which has a "std::transform" function that can iterate through a container and apply a conversion function to each element. - For example, in Lab04 we wanted to convert a string to all upper case characters.
- There are many ways to do this…
- And since strings are iterable, we can apply a function to each item (i.e. char) in the string.
Example (using std::transform)
#include <iostream>
#include <string>
#include <algorithm>
using namespace std;
int main() {
string s = "abcd";
cout << s << endl;
transform(s.begin(), s.end(), s.begin(), ::toupper);
cout << s << endl;
}
// Output:
// abcd
// ABCD
- First thing to observe is what the transform function does:
- 1st parameter: The starting position of where to iterate through.
- 2nd parameter: The end position where to stop iterating.
- 3rd parameter: The start position of where to replace content in the container.
- 4th parameter: The function to apply on each element when iterating.
transform(s.begin() + 2, s.end(), s.begin(), ::toupper);
// Output:
// abcd
// CDcd
- The second thing to observe is the
toupperfunction.- Takes in a single char in its parameter, which is what the iterator of a string is referencing.
- If
touppertook in a type different than a char (or had additional required parameters), then this would not work. ::toupperis required in this case sincetoupperexists in thestdnamespace.- Using
::toupperlooks for the toupper function in the global namespace, which is whattransformis expecting.
- Using
Another example defining our own function for the transform function
// Our defined function
int multiplyBy100(int x) { return 100 * x; }
// Function to print contents of the vector
void printVector(vector<int> &v) {
for (int i = 0; i < v.size(); i++)
cout << "v[" << i << "] = " << v[i] << endl;
cout << "---" << endl;
}
int main() {
vector<int> v;
for (int i = 10; i <= 15; i++) {
v.push_back(i);
}
printVector(v);
transform(v.begin(), v.end(), v.begin(), multiplyBy100); // uses our function
printVector(v);
return 0;
}
- So we are passing functions in parameters and calling them.
- callbacks is a term referring to passing an executable segment of code to another segment of code.
- Under-the-hood, functions are stored in the executable and can be located in a specific memory address.
- We can find the memory address of a function by using
reinterpret_cast
cout << multiplyBy100 << endl; // warning: prints 1 (but does compile correctly)
cout << reinterpret_cast<void*>(multiplyBy100) << endl; // prints an address of where the multiplyBy100 exists in memory
- We can start defining our own pointers to functions and assign these pointers to functions’ memory address.
Example defining function pointers
int main() {
// Pointer to a function that has an int parameter and returns an int.
int (*someFunctionPointer)(int);
// Assign the address of multiplyBy100 to the function pointer.
someFunctionPointer = multiplyBy100;
// ERROR!!, trying to assign an int to a function pointer
// someFunctionPointer = multiplyBy100(10);
// Using the function pointer to call a function.
// Dereferences the pointer as a function, and then passes the parameter
// 2 to the function.
cout << (*someFunctionPointer)(2) << endl;
// syntactically OK to use the function pointer directly without
// dereferencing it
cout << someFunctionPointer(2) << endl; // OK
// ERROR!! Order of operations dereferences after attempting to call the
// function
// cout << *someFunctionPointer(2) << endl;
}
Passing a function to another function as a parameter
- Similar to the transform function – a function can take a function pointer as one of its parameters and use whatever function the function pointer is referring to.
Example: Writing a template filter function for a vector
template <class T>
void printVector(vector<T> &v) {
for (int i = 0; i < v.size(); i++)
cout << "v[" << i << "] = " << v[i] << endl;
cout << "---" << endl;
}
template <class T>
bool notEqual(T x, T y) {
return x != y;
}
template<class T>
vector<T> filterVector(vector<T> v, T value, bool (*filterFunc)(T, T)) {
vector<T> result;
for (int i = 0; i < v.size(); i++) {
if (filterFunc(v[i], value)) {
result.push_back(v[i]);
}
}
return result;
}
int main() {
// vector of strings
vector<string> v1;
v1.push_back("s1");
v1.push_back("s2");
v1.push_back("s3");
v1.push_back("s2");
v1.push_back("s1");
string remove = "s2";
vector<string> result1 = filterVector(v1, remove, notEqual);
printVector(result1);
// vector of ints
vector<int> v2;
v2.push_back(1);
v2.push_back(2);
v2.push_back(3);
v2.push_back(2);
v2.push_back(1);
vector<int> result2 = filterVector(v2, 1, notEqual);
printVector(result2);
return 0;
}