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Let the Compiler Deduce a Type

Let the Compiler Deduce a Type

Auto asks the compiler to deduce a variable’s type from its initializer.

It reduces repetition without turning C++ into a dynamically typed language.

Before this lesson, review Convert Values Deliberately when needed.

What you’ll learn

  • Use auto to let the Compiler Deduce a Type.
  • Explain how the complete example produces 75.
  • Make the practice change and explain why the result changes or stays the same.

Follow a complete C++ example

It reduces repetition without turning C++ into a dynamically typed language.

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>

int main()  {
  auto marks = 72;
  marks += 3;
  std::cout << marks << '\n';
  return 0;
}

Expected output

75
Run in C++ compiler →

Connect the code to its result

C++17 · source used in this explanation

#include <iostream>

int main()  {
  auto marks = 72;
  marks += 3;
  std::cout << marks << '\n';
  return 0;
}

Expected output

75

Auto asks the compiler to deduce a variable’s type from its initializer. It reduces repetition without turning C++ into a dynamically typed language.

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 75. In this example, distinguish what the program calculates or changes from what it actually displays.

Important mistakes and boundaries

The deduced type remains fixed. Auto can copy a value; auto& instead declares a reference when deduction permits it.

Change the school example

C++17 · edit this complete example

#include <iostream>

int main()  {
  auto marks = 72;
  marks += 3;
  std::cout << marks << '\n';
  return 0;
}

Your task: Change the starting mark to 80. 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: Change the starting mark to 80. The final value is 83.

Auto asks the compiler to deduce a variable’s type from its initializer. It reduces repetition without turning C++ into a dynamically typed language.

The deduced type remains fixed. Auto can copy a value; auto& instead declares a reference when deduction permits it.

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>

int main()  {
  auto marks = 80;
  marks += 3;
  std::cout << marks << '\n';
  return 0;
}

Expected output

83
Run in C++ compiler →

Check your understanding

C++17 · original program for this question

#include <iostream>

int main()  {
  auto marks = 72;
  marks += 3;
  std::cout << marks << '\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

75

Auto asks the compiler to deduce a variable’s type from its initializer. It reduces repetition without turning C++ into a dynamically typed language. The deduced type remains fixed. Auto can copy a value; auto& instead declares a reference when deduction permits it.

What to remember

Auto asks the compiler to deduce a variable’s type from its initializer. The deduced type remains fixed. Auto can copy a value; auto& instead declares a reference when deduction permits it.

Useful connections: Convert Values Deliberately, 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: Calculate Totals and Remainders →