Read Without Copying or Changing
Read Without Copying or Changing
A const reference lets a function read an existing object without copying it or modifying it through that parameter.
This suits a student-name display routine.
Before this lesson, review Refer to an Existing Value when needed.
What you’ll learn
- Use const references to read Without Copying or Changing.
- Explain how the complete example produces 4.
- Make the practice change and explain why the result changes or stays the same.
Follow a complete C++ example
This suits a student-name display routine.
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::size_t nameLength(const std::string& name) {
return name.size();
}
int main() {
std::string name = "Diya";
std::cout << nameLength(name) << '\n';
return 0;
}
Expected output
4Connect the code to its result
C++17 · source used in this explanation
#include <iostream>
#include <string>
std::size_t nameLength(const std::string& name) {
return name.size();
}
int main() {
std::string name = "Diya";
std::cout << nameLength(name) << '\n';
return 0;
}
Expected output
4A const reference lets a function read an existing object without copying it or modifying it through that parameter. This suits a student-name display routine.
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 4. In this example, distinguish what the program calculates or changes from what it actually displays.
Important mistakes and boundaries
Const access through one reference does not freeze changes through every other alias. A returned reference must not outlive the object it refers to.
Change the school example
C++17 · edit this complete example
#include <iostream>
#include <string>
std::size_t nameLength(const std::string& name) {
return name.size();
}
int main() {
std::string name = "Diya";
std::cout << nameLength(name) << '\n';
return 0;
}
Your task: Change the name to Saira. 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 name to Saira. Its char-element count is 5.
A const reference lets a function read an existing object without copying it or modifying it through that parameter. This suits a student-name display routine.
Const access through one reference does not freeze changes through every other alias. A returned reference must not outlive the object it refers to.
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::size_t nameLength(const std::string& name) {
return name.size();
}
int main() {
std::string name = "Saira";
std::cout << nameLength(name) << '\n';
return 0;
}
Expected output
5Check your understanding
C++17 · original program for this question
#include <iostream>
#include <string>
std::size_t nameLength(const std::string& name) {
return name.size();
}
int main() {
std::string name = "Diya";
std::cout << nameLength(name) << '\n';
return 0;
}
What to remember
A const reference lets a function read an existing object without copying it or modifying it through that parameter. Const access through one reference does not freeze changes through every other alias. A returned reference must not outlive the object it refers to.
Useful connections: Refer to an Existing Value, 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.