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Choose by Parameter Types

Choose by Parameter Types

Overloading allows one function name with different parameter lists.

The compiler selects a suitable overload from the call’s argument types, not from a desired return value.

Before this lesson, review Separate Visibility from Lifetime when needed.

What you’ll learn

  • Use function overloading to choose by Parameter Types.
  • Explain how the complete example produces Whole mark → Decimal mark.
  • Make the practice change and explain why the result changes or stays the same.

Follow a complete C++ example

The compiler selects a suitable overload from the call’s argument types, not from a desired return value.

Read the declarations above main before tracing its statements. Each declared name supplies the value, type or operation used later. The std:: prefix selects a name from the standard library, and cout << sends a value to the output stream.

C++17 · main.cpp · complete program

#include <iostream>
#include <string>

std::string grade(int)  {
  return "Whole mark";
}
std::string grade(double)  {
  return "Decimal mark";
}
int main()  {
  std::cout << grade(72) << '\n' << grade(72.5) << '\n';
  return 0;
}

Expected output

Whole mark
Decimal mark
Run in C++ compiler →

Connect the code to its result

C++17 · source used in this explanation

#include <iostream>
#include <string>

std::string grade(int)  {
  return "Whole mark";
}
std::string grade(double)  {
  return "Decimal mark";
}
int main()  {
  std::cout << grade(72) << '\n' << grade(72.5) << '\n';
  return 0;
}

Expected output

Whole mark
Decimal mark

Overloading allows one function name with different parameter lists. The compiler selects a suitable overload from the call’s argument types, not from a desired return value.

Trace the result: Read the complete program below from its starting values to the output operations. Each std::cout statement displays the value supplied at that point; \n begins the next line. The resulting lines, in order, are Whole mark → Decimal mark. In this example, distinguish what the program calculates or changes from what it actually displays.

Important mistakes and boundaries

Changing only the return type does not create a valid overload. Conversions can make overload choices surprising or ambiguous.

Change the school example

C++17 · edit this complete example

#include <iostream>
#include <string>

std::string grade(int)  {
  return "Whole mark";
}
std::string grade(double)  {
  return "Decimal mark";
}
int main()  {
  std::cout << grade(72) << '\n' << grade(72.5) << '\n';
  return 0;
}

Your task: Use grade(80.5) once. Before running your edited program, predict the new output and explain which stored value, condition or object changes. Keep the rest of the example intact.

Use the complete source repeated here; this task does not depend on remembering code from another section.

Worked answer and reasoning

Worked variation: Use grade(80.5) once. The decimal overload returns Decimal mark.

Overloading allows one function name with different parameter lists. The compiler selects a suitable overload from the call’s argument types, not from a desired return value.

Changing only the return type does not create a valid overload. Conversions can make overload choices surprising or ambiguous.

For a baseline run, save this source as main.cpp and use clang++ -std=c++17 -Wall -Wextra -pedantic main.cpp -o school, or equivalent g++ flags. Recompile after editing so that you run the updated executable.

The source and output below belong to the edited program. Compare them with the original example to see the consequence of this single change.

C++17 · worked variation · complete program

#include <iostream>
#include <string>

std::string grade(int)  {
  return "Whole mark";
}
std::string grade(double)  {
  return "Decimal mark";
}
int main()  {
  std::cout << grade(72) << '\n' << grade(80.5) << '\n';
  return 0;
}

Expected output

Whole mark
Decimal mark
Run in C++ compiler →

Check your understanding

C++17 · original program for this question

#include <iostream>
#include <string>

std::string grade(int)  {
  return "Whole mark";
}
std::string grade(double)  {
  return "Decimal mark";
}
int main()  {
  std::cout << grade(72) << '\n' << grade(72.5) << '\n';
  return 0;
}
Before making the practice edit, what does the original C++17 program print?
Choose one answer. You can try again.

Answer and explanation

Whole mark Decimal mark

Overloading allows one function name with different parameter lists. The compiler selects a suitable overload from the call’s argument types, not from a desired return value. Changing only the return type does not create a valid overload. Conversions can make overload choices surprising or ambiguous.

What to remember

Overloading allows one function name with different parameter lists. Changing only the return type does not create a valid overload. Conversions can make overload choices surprising or ambiguous.

Useful connections: Separate Visibility from Lifetime, Give a Task a Name.

Language and design guidance: C++ Core Guidelines and C++ working draft. The draft describes an evolving language; these examples deliberately target C++17.

Reaching the recap and completing the quizzes are separate milestones.

    Next lesson: Solve a Smaller Version →