---
description: "Learn more about: C++ Constant Expressions"
title: "C++ Constant Expressions"
ms.date: "11/04/2016"
helpviewer_keywords: ["constant expressions, syntax", "constant expressions", "expressions [C++], constant"]
ms.assetid: b07245a5-4c21-4589-b503-e6ffd631996f
---
# C++ Constant Expressions

A *constant* value is one that doesn't change. C++ provides two keywords to enable you to express the intent that an object is not intended to be modified, and to enforce that intent.

C++ requires constant expressions — expressions that evaluate to a constant — for declarations of:

- Array bounds

- Selectors in case statements

- Bit-field length specification

- Enumeration initializers

The only operands that are legal in constant expressions are:

- Literals

- Enumeration constants

- Values declared as const that are initialized with constant expressions

- **`sizeof`** expressions

Nonintegral constants must be converted (either explicitly or implicitly) to integral types to be legal in a constant expression. Therefore, the following code is legal:

```cpp
const double Size = 11.0;
char chArray[(int)Size];
```

Explicit conversions to integral types are legal in constant expressions; all other types and derived types are illegal except when used as operands to the **`sizeof`** operator.

The comma operator and assignment operators cannot be used in constant expressions.

## See also

[Types of Expressions](../cpp/types-of-expressions.md)