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

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); }

thanks, can it implement using some semantics like const casting.
@amont Can you clarify, please, which semantics for example? const_cast generally is a red flag for bad design in most cases.
Beware: using auto as parameter creates a templated function. To save repeated typing an alias with using could be created instead.
On my complier, I can't transfer function parameter as auto.
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

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" } };

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