# Projection Functions

Learn how to use projection functions to apply range-based algorithms on derived data
This lesson is part of theÂ course:

### Professional C++

Comprehensive course covering advanced concepts, and how to use them on large-scale projects.

Free, Unlimited Access
###### Ryan McCombe
Updated

When using an algorithm that acts on items in a collection, we often don't want the exact values in the collection to be theÂ input.

For example, we might have a collection of numbers that we want to sort by their absoluteÂ value.

Or, we have a collection of Player objects, and we want to run an algorithm on a property of those objects, such as their emailÂ addresses.

## Projection Functions

To generalize the above ideas, we want our algorithms to receive projections of the objects in our collection. Each projection is based on the original object but can have a different value and even a differentÂ type.

To support this, almost every range-based algorithm in the <ranges> library has an overload that receives a projectionÂ function.

These functions will receive objects within our collection as a parameter and will return a new object based on thatÂ parameter.

The algorithm will then use these projections to drive the behavior of theÂ algorithm.

## Example: Sorting Numbers By Absolute Value

In the previous lesson, we introduced the std::ranges::sort() algorithm. Its first argument is the range we want to sort, and the optional second argument is the comparison function to use for sorting theÂ range.

In this example, we pass {} as the comparison function, causing the algorithm to use the defaultÂ value:

#include <algorithm>
#include <iostream>
#include <vector>

int main() {
std::vector Nums{-3, 5, 0};
std::ranges::sort(Nums, {});
for (auto Num : Nums) {
std::cout << Num << ", ";
}
}
-3, 0, 5,

The std::ranges::sort() function also has an optional third parameter, which is how we provide a projectionÂ function.

In the next example, we pass a function that projects the numbers in our collection to their absolute value, using std::abs():

#include <algorithm>
#include <iostream>
#include <vector>

int Project(int x) {
return std::abs(x);
}

int main() {
std::vector Nums{-3, 5, 0};
std::ranges::sort(Nums, {}, Project);
for (auto Num : Nums) {
std::cout << Num << ", ";
}
}

Now, our std::ranges::sort() call has sorted our numbers by their projection - that is, their absoluteÂ value:

0, -3, 5,

### Why not just use the comparison function for this?

With the std::ranges::sort() algorithm, we could also have implemented this behavior by customizing the comparisonÂ functions:

#include <algorithm>
#include <iostream>
#include <vector>

int main() {
std::vector Nums{-3, 5, 0};

std::ranges::sort(Nums, [](int a, int b){
return std::abs(a) < std::abs(b);
});

for (auto Num : Nums) {
std::cout << Num << ", ";
}
}
0, -3, 5,

But, the comparison function is specific to std::ranges::sort() - this is not a technique that we can alwaysÂ use.

Projection functions, meanwhile, are available to almost every algorithm in std::ranges.

## Example: Projection to a Different Type

Our projection function does not need to return the same type of object that was contained in our originalÂ collection.

In this example, we sort Player objects by level, using a projectionÂ function:

#include <vector>
#include <iostream>
#include <algorithm>

struct Player {
std::string Name;
int Level;
};

int main() {
std::vector Party {
Player {"Legolas", 49},
Player {"Gimli", 47},
Player {"Gandalf", 53}
};

std::ranges::sort(Party, {}, [](Player& P) {
return P.Level;
});

for (const auto& P : Party) {
std::cout << "[" << P.Level << "] "
<< P.Name << "\n";
}
}
[47] Gimli
[49] Legolas
[53] Gandalf

Here, we combine both a projection and a comparison function. The projection function will return an int, and then the comparison function will compare those intÂ values:

#include <vector>
#include <iostream>
#include <algorithm>

struct Player {/*...*/}

int main() {
std::vector Party {/*...*/}

std::ranges::sort(
Party,
[](int a, int b) { return a > b; },
[](Player& P) { return P.Level; }
);

for (const auto& P : Party) {/*...*/}
}
[53] Gandalf
[49] Legolas
[47] Gimli

## Example: Projection to a Class Member

Finally, we often want our algorithms to use a class member as theirÂ projection.

In such cases, we can simply pass a reference to that function. Below, we use Player::GetName as our projection function, causing our Player objects to be sorted alphabetically byÂ name:

#include <vector>
#include <iostream>
#include <algorithm>

class Player {/*...*/}

int main() {
std::vector Party {
Player{"Legolas"},
Player{"Gimli"},
Player{"Gandalf"}
};

std::ranges::sort(Party, {}, &Player::GetName);

for (const auto& P : Party) {
std::cout << P.GetName() << '\n';
}
}
[53] Gandalf
[47] Gimli
[49] Legolas

## Summary

In this lesson, we've explored how to utilize projection functions with C++'s <ranges> library to manipulate and sort collections based on their inherent and derived properties. These techniques allow for more flexible and powerful data manipulation, enhancing the versatility of range-basedÂ algorithms.

### Main Points Covered

• Projection functions allow algorithms to operate on transformed or derived values from the objects in a collection.
• The <ranges> library supports projection in almost all range-based algorithms, enabling operations on both value and type-transformed projections.
• Examples demonstrated sorting numbers by their absolute value and sorting objects based on a member's value, showcasing the utility of projections.
• We compared the use of projection functions with custom comparison functions, highlighting projections' broader applicability across different algorithms.
• The lesson highlighted the syntax and usage of projection functions, including how to apply them to sort collections by a property or even by a member function's return value.

Next Lesson

### Standard Library Views

Learn how to create and use views in C++ using examples from std::views
New: AI-Powered AssistanceAI Assistance

### Questions and HelpNeed Help?

Get instant help using our free AI assistant, powered by state-of-the-art language models.

Updated
Lesson Contents

### Projection Functions

Learn how to use projection functions to apply range-based algorithms on derived data

This lesson is part of theÂ course:

### Professional C++

Comprehensive course covering advanced concepts, and how to use them on large-scale projects.

Free, Unlimited Access

###### Projection Functions

Learn how to use projection functions to apply range-based algorithms on derived data

This lesson is part of the course:

## Professional C++

Comprehensive course covering advanced concepts, and how to use them on large-scale projects.

Free, unlimited access

### This course includes:

• 124 Lessons
• 550+ Code Samples
• 96% Positive Reviews
• Regularly Updated
• Help and FAQ
Next Lesson

### Standard Library Views

Learn how to create and use views in C++ using examples from std::views