add a few new programs related to templates

i made new programs related to lambdas, and various things you can do
with templates like generic functions and overriding the () operator
meaning you can call a class object like a function.

src/CMakeLists.txt

@@ -20,3 +20,6 @@ add_executable(vector vector.cc)
 add_executable(Vector2 Vector2.cc)
 add_executable(TemplateVector TemplateVector.cc)
 add_executable(buffer buffer.cc)
+add_executable(FunctionTemplates FunctionTemplates.cc)
+add_executable(FunctionObjects FunctionObjects.cc)
+add_executable(lambdas lambdas.cc)

src/FunctionObjects.cc

@@ -0,0 +1,54 @@
+// you can override the () operator meaning you can call class objects like functions.
+
+#include <format>
+#include <iostream>
+#include <string>
+
+template <typename T>
+class LessThan {
+private:
+	const T val;
+
+public:
+	LessThan(const T& v)
+		: val { v }
+	{
+	}
+
+	bool operator()(const T& x) const
+	{
+		return x < val;
+	}
+};
+
+template <typename T>
+class GreaterThan {
+private:
+	const T val;
+
+public:
+	GreaterThan(const T& v)
+		: val { v }
+	{
+	}
+
+	bool operator()(const T& x) const
+	{
+		return x > val;
+	}
+};
+
+int main()
+{
+	LessThan<int> i1(21);
+	LessThan<double> i2(21.420);
+
+	GreaterThan<int> i3(21);
+	GreaterThan<double> i4(21.420);
+
+	std::cout << std::format("less than 21: {}", i1(1)) << std::endl;
+	std::cout << std::format("less than 21.420: {}", i2(420.21)) << std::endl;
+
+	std::cout << std::format("greater than 21: {}", i3(1)) << std::endl;
+	std::cout << std::format("greater than 21.420: {}", i4(420.21)) << std::endl;
+}

src/FunctionTemplates.cc

@@ -0,0 +1,28 @@
+#include <format>
+#include <iostream>
+#include <string>
+
+template <typename T>
+T sum(const T& sum1, const T& sum2)
+{
+	return sum1 + sum2;
+}
+
+void print_number(const std::string& stringName, const auto& value)
+{
+	std::cout << std::format("{} = {}", stringName, value) << std::endl;
+}
+
+int main()
+{
+	double i1 = sum(1.4, 1.2);
+	int i2 = sum(1, 1);
+
+	auto i3 = sum<int>(2, 2);
+	auto i4 = sum<double>(4.4, 4.2);
+
+	print_number("i1", i1);
+	print_number("i2", i2);
+	print_number("i3", i3);
+	print_number("i4", i4);
+}

src/lambdas.cc

@@ -0,0 +1,23 @@
+#include <format>
+#include <iostream>
+
+template <typename T>
+bool less_than(const T& i)
+{
+	return [&]() {
+		return i < 21;
+	}();
+}
+
+auto greater_than = [](const double i) {
+	return i > 21.420;
+};
+
+int main()
+{
+	std::cout << std::format("less than 21 <lambda edition>: {}", less_than(20)) << std::endl;
+	std::cout << std::format("less than 21 <lambda edition>: {}", less_than(22)) << std::endl;;
+
+	std::cout << std::format("greater than 21 <lambda edition>: {}", less_than(22)) << std::endl;
+	std::cout << std::format("greater than 21 <lambda edition>: {}", less_than(20)) << std::endl;
+}