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Reuse a Typed Container

Reuse a Typed Container

A class template describes related types with one design.

A small Box can store an integer mark while Box uses the same field structure for a name.

Before this lesson, review Reuse a Typed Calculation when needed.

What you’ll learn

  • Use class templates to reuse a Typed Container.
  • Explain how the complete example produces 72 → Diya.
  • Make the practice change and explain why the result changes or stays the same.

Follow a complete C++ example

A small Box can store an integer mark while Box uses the same field structure for a name.

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>

template<typename T> struct Box  {
  T value;
}
;
int main()  {
  Box<int> mark {
    72
  }
  ;
  Box<std::string> name {
    "Diya"
  }
  ;
  std::cout << mark.value << '\n' << name.value << '\n';
  return 0;
}

Expected output

72
Diya
Run in C++ compiler →

Connect the code to its result

C++17 · source used in this explanation

#include <iostream>
#include <string>

template<typename T> struct Box  {
  T value;
}
;
int main()  {
  Box<int> mark {
    72
  }
  ;
  Box<std::string> name {
    "Diya"
  }
  ;
  std::cout << mark.value << '\n' << name.value << '\n';
  return 0;
}

Expected output

72
Diya

A class template describes related types with one design. A small Box can store an integer mark while Box uses the same field structure for a name.

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

Important mistakes and boundaries

Template definitions usually need to be visible where instantiated. A template is not a runtime container that silently changes its element type.

Change the school example

C++17 · edit this complete example

#include <iostream>
#include <string>

template<typename T> struct Box  {
  T value;
}
;
int main()  {
  Box<int> mark {
    72
  }
  ;
  Box<std::string> name {
    "Diya"
  }
  ;
  std::cout << mark.value << '\n' << name.value << '\n';
  return 0;
}

Your task: Use Box with 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: Use Box with 80. The mark line becomes 80.

A class template describes related types with one design. A small Box can store an integer mark while Box uses the same field structure for a name.

Template definitions usually need to be visible where instantiated. A template is not a runtime container that silently changes its element type.

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>

template<typename T> struct Box  {
  T value;
}
;
int main()  {
  Box<int> mark {
    80
  }
  ;
  Box<std::string> name {
    "Diya"
  }
  ;
  std::cout << mark.value << '\n' << name.value << '\n';
  return 0;
}

Expected output

80
Diya
Run in C++ compiler →

Check your understanding

C++17 · original program for this question

#include <iostream>
#include <string>

template<typename T> struct Box  {
  T value;
}
;
int main()  {
  Box<int> mark {
    72
  }
  ;
  Box<std::string> name {
    "Diya"
  }
  ;
  std::cout << mark.value << '\n' << name.value << '\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

72 Diya

A class template describes related types with one design. A small Box<int> can store an integer mark while Box<std::string> uses the same field structure for a name. Template definitions usually need to be visible where instantiated. A template is not a runtime container that silently changes its element type.

What to remember

A class template describes related types with one design. Template definitions usually need to be visible where instantiated. A template is not a runtime container that silently changes its element type.

Useful connections: Reuse a Typed Calculation, 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: Represent a Position in a Range →