How to const reference an object in the c++ for range loop

Question

In the following code, [id, name] is a const reference. However, studentMap is non-const. The user can change the value of studentMap in the loop. I want to ask whether there is a way to make the StudentMap also const. Thanks.

#include <iostream>
#include <string>
#include <map>


int main() {
    std::map<int, std::string> studentMap;
    studentMap[1] = "Tom";
    studentMap[7] = "Jack";
    studentMap[15] = "John";

    for (const auto& [id, name] : studentMap) {
        studentMap.at(id) += "test";
    }

    for (const auto& [id, name]: studentMap) {
        std::cout << id << " " << name << "\n";
    }

    return 0;
}

You need studentMap to be non-const as you are setting that up before the loops. So, you could do something like creating a new scope (e.g. a new function), have a const ref referring to studentMap, and iterate thru that constStudentMap — Adrian Shum
– Adrian Shum, Commented May 5, 2022 at 3:33
You can also create a new variable instead of a new function: const auto& constStudentMap = studentMap; — Sebastian
– Sebastian, Commented May 5, 2022 at 3:57

Adrian Shum · Accepted Answer · 2022-05-05 03:40:03Z

3

No I don't think it is possible to change the type of a variable.

If you want to avoid unexpected mistake of modifying studentMap, you could pull the logic into a separate function, and refer studentMap with a const ref:

#include <iostream>
#include <string>
#include <map>

void displayStudentMap(const auto& studentMap) {
    for (const auto& [id, name] : studentMap) {
        // compilation error
        studentMap.at(id) += "test";
    }

    for (const auto& [id, name]: studentMap) {
        std::cout << id << " " << name << "\n";
    }

}

int main() {
    std::map<int, std::string> studentMap;
    studentMap[1] = "Tom";
    studentMap[7] = "Jack";
    studentMap[15] = "John";

    displayStudentMap(studentMap);
}

answered May 5, 2022 at 3:40

Adrian Shum

40.3k11 gold badges89 silver badges134 bronze badges

Sign up to request clarification or add additional context in comments.

10 Comments

amont Over a year ago

thanks, can it implement using some semantics like const casting.

Sebastian Over a year ago

@amont Can you clarify, please, which semantics for example? const_cast generally is a red flag for bad design in most cases.

Sebastian Over a year ago

Beware: using auto as parameter creates a templated function. To save repeated typing an alias with using could be created instead.

amont Over a year ago

On my complier, I can't transfer function parameter as auto.

David C. Rankin Over a year ago

C++ does not allow auto in parameter declarations -- that is provided by a non-standard compiler extension. To be compliant you want std::map<int, std::string>& studentMap You can't pass as const if you modify with += "test"; Both are easily fixed by either removing const or not modifying studentMap in the function.

|

Alvov1 · Accepted Answer · 2024-01-20 18:19:11Z

3

This way:

const std::map<int, std::string> studentMap {
    std::make_pair(1, "Tom"),
    std::make_pair(7, "Jack"),
    std::make_pair(15, "John")
};

or this with C++11 uniform initialization:

const std::map<int, std::string> studentMap {
        { 1, "Tom" },
        { 7, "Jack" },
        { 15, "John" }
};

edited Jan 20, 2024 at 18:19

answered May 5, 2022 at 8:59

Alvov1

3271 gold badge3 silver badges16 bronze badges

Collectives™ on Stack Overflow

How to const reference an object in the c++ for range loop

2 Answers 2

10 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

10 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related