The 'Expression Must Have Integral or Unscoped Enum' error is a common error encountered by developers while writing code. This error typically arises when the developer uses a non-integral or non-unscoped enumeration type in a switch statement, a loop, or as an array size specifier. In this guide, we will discuss the reasons behind this error and provide step-by-step solutions to resolve it.
Table of Contents
The Cause of the Error
In programming languages such as C++ and C#, only integral types (like int, char, short, and long) and unscoped enumerations are allowed to be used in certain contexts, like switch statements, loops, or determining the size of an array. If you try to use a non-integral type or a scoped enumeration (like enum class in C++), the compiler will throw the 'Expression Must Have Integral or Unscoped Enum' error.
For example, in C++, the following code will generate this error:
enum class Color { Red, Green, Blue };
void foo(Color color) {
switch (color) {
case Color::Red:
// ...
break;
case Color::Green:
// ...
break;
case Color::Blue:
// ...
break;
}
}
This is because Color is a scoped enumeration, and as such, it cannot be used in a switch statement.
Resolving the Error
To resolve the 'Expression Must Have Integral or Unscoped Enum' error, you need to ensure that you are using an integral type or an unscoped enumeration in the respective programming construct.
Switch Statements
If you encounter this error while using a switch statement, you can resolve it by converting the scoped enumeration to an unscoped enumeration, or by using an integral value to represent the enumeration values.
For instance, in the above example, you can change the enum class to a regular enum to resolve the error:
enum Color { Red, Green, Blue };
void foo(Color color) {
switch (color) {
case Red:
// ...
break;
case Green:
// ...
break;
case Blue:
// ...
break;
}
}
Alternatively, you can use an integral value to represent the enumeration values in the switch statement:
enum class Color { Red, Green, Blue };
void foo(Color color) {
switch (static_cast<int>(color)) {
case static_cast<int>(Color::Red):
// ...
break;
case static_cast<int>(Color::Green):
// ...
break;
case static_cast<int>(Color::Blue):
// ...
break;
}
}
Loops and Array Sizes
If you encounter this error while using a loop or specifying an array size, you can resolve it by ensuring that you use an integral type for the loop counter or array size.
For example, if you have the following code in C++:
enum class Size { Small, Medium, Large };
void foo() {
int arr[Size::Large];
}
You can resolve the error by using the integer value of the enumeration:
enum class Size { Small, Medium, Large };
void foo() {
int arr[static_cast<int>(Size::Large)];
}
FAQ
1. What is an unscoped enumeration in C++?
An unscoped enumeration is a traditional C++ enumeration declared with the enum keyword. The enumeration values are not scoped, meaning they can be accessed without specifying the enumeration name.
2. What is a scoped enumeration in C++?
A scoped enumeration is a C++11 feature declared with the enum class keyword. The enumeration values are scoped, meaning they must be accessed by specifying the enumeration name.
3. Can I use a floating-point type in a switch statement?
No, only integral types and unscoped enumerations are allowed in switch statements.
4. Can I use a std::size_t type for array sizes?
Yes, std::size_t is an integral type, and it is the recommended type for specifying array sizes and indices.
5. Can I use a bool type in a loop?
Yes, a bool type can be used as a loop condition, but it cannot be used as a loop counter.