| Issue |
179064
|
| Summary |
clang-tidy check proposal: suggest-missing-const
|
| Labels |
clang-tidy
|
| Assignees |
|
| Reporter |
Saint-Martin
|
## 1. Introduction
The `const` keyword greatly improves the readability and correctness of C++ software: it clarifies APIs, helps static analysis, and prevents accidental modifications (Scott Meyers, *Effective C++*: “use const whenever possible”).
However, developers may sometimes not see, or forget to mark, a variable or a member function as `const` even when it could be.
A static analysis tool, such as clang-tidy, could help by detecting these missed opportunities.
## 2. Motivation
Help the programmer identify variables and functions that could safely be declared `const`.
## 3. Problem Description
Examples of missing `const`:
- A local variable is never written after initialization
`int a = 3;`
- A parameter is not modified and could be `const`
`int f(const int& n) { int m = n; return 0; }`
- A member function does not modify any member and could be `const`
``
struct A {
int x;
void f() { std::cout << x; } // could be const
};
``
- A global variable is never written after initialization
## 4. Proposed Solution
clang-tidy could detect these missing `const` qualifiers and warn the programmer.
### 4.1 Heuristic
1. **Missing const variable**
The variable is not written to after initialization within its scope.
2. **Missing const method**
- The method does not write to any non-mutable member
- The method only calls other `const` methods
- (This heuristic does not consider global state or side effects.)
### 4.2 Limitations
- Does not detect transitive writes through pointers or references
- Does not consider global state
- Does not infer logical constness
## 5. Examples
### Example 1 — Detecting variables that could have been const
```
#include <iostream>
int main() {
int term = 2; // should emit: "variable 'term' could be declared const"
int tab[] = {0, 1, 2};
for (int i = 0; i != 3; ++i) {
int x = i + term;
}
return 0;
}
```
### Example 2 — Detecting member functions that could have been const
```
#include <iostream>
class B {
public:
void write() const {
std::cout << "B writing" << std::endl;
}
};
class A {
B b;
public:
void write() { // write could have been const
b.write();
}
};
```
## 6. Impact
Improves readability, maintainability, and debugging clarity.
## 7. Conclusion
Feedback and suggestions are welcome and appreciated.
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