The error “expression must have a constant value” in C++ length occurs when you try to use a non-constant value in a context that requires a constant expression.
To fix the “expression must have a constant value” in C++ length error, declare it constant with the const keyword. You can also use the constexpr specifier introduced in C++11 that declares that it is possible to evaluate the value of the function or variable at compile time.
Such variables and functions can then be used where only compile-time constant expressions are allowed.
const int value = 10;
constexpr int result = value * 2;Why the “expression must have a constant value” error occurs?
The “expression must have a constant value” error occurs in C++ when you try to use a non-constant expression in a context that requires a constant expression.
A constant expression is an expression that can be evaluated at compile time, and its value does not change during the execution of the program.
In C++, arrays must have a constant size, so the size of an array must be a constant expression. Similarly, the case labels in a switch statement must also be constant expressions.
The expression must be evaluated at compile-time and have a constant value in both cases.
We already saw the solution at the beginning of the article, but if your error persists, you can go through the below alternate solutions.
Use a constant expression in a constant expression context to fix the error
If you use a non-constant expression in an array size or a switch statement case label, you can replace it with a constant expression.
When creating an array, its size must be constant.
If you want a dynamically sized array, you need to allocate memory for it on the heap and free it with delete when you are done.
int** arr = new int*[row];
for(int i = 0; i < row; i++)
arr[i] = new int[col];
// use the array
//deallocate the array
for(int i = 0; i < row; i++)
delete[] arr[i];
delete[] arr;The code creates a two-dimensional array of int values using dynamic memory allocation in C++.
The new operator is used to allocate memory dynamically, and the delete operator is used to deallocate the memory when it is no longer needed.
The code creates an array of row number pointers to int values and assigns each of these pointers to a new array of col int values. This creates a two-dimensional array where arr[i] is an array of col int values, and arr[i][j] is a single int value.
After using the array, we deallocated the memory by deallocating each sub-arrays using the delete[] operator and then deallocating the main array using the delete[] operator. This assures that all dynamically allocated memory is freed and prevents memory leaks.
It’s important to always properly deallocate dynamic memory to avoid memory leaks and ensure the stability and efficiency of your program.
I hope this resolves your error.
