---
title: "static_assert"
ms.date: "07/29/2019"
f1_keywords: ["static_assert_cpp"]
helpviewer_keywords: ["assertions [C++], static_assert", "static_assert"]
ms.assetid: 28dd3668-e78c-4de8-ba68-552084743426
---
# static_assert

Tests a software assertion at compile time. If the specified constant expression is FALSE, the compiler displays the specified message, if one is provided, and the compilation fails with error C2338; otherwise, the declaration has no effect.

## Syntax

```
static_assert( constant-expression, string-literal );

static_assert( constant-expression ); // C++17 (Visual Studio 2017 and later)
```

#### Parameters

|Parameter|Description|
|---------------|-----------------|
|*constant-expression*|An integral constant expression that can be converted to a Boolean.<br /><br /> If the evaluated expression is zero (false), the *string-literal* parameter is displayed and the compilation fails with an error. If the expression is nonzero (true), the **static_assert** declaration has no effect.|
|*string-literal*|An message that is displayed if the *constant-expression* parameter is zero. The message is a string of characters in the [base character set](../c-language/ascii-character-set.md) of the compiler; that is, not [multibyte or wide characters](../c-language/multibyte-and-wide-characters.md).|

## Remarks

The *constant-expression* parameter of a **static_assert** declaration represents a *software assertion*. A software assertion specifies a condition that you expect to be true at a particular point in your program. If the condition is true, the **static_assert** declaration has no effect. If the condition is false, the assertion fails, the compiler displays the message in *string-literal* parameter, and the compilation fails with an error. In Visual Studio 2017 and later, the string-literal parameter is optional.

The **static_assert** declaration tests a software assertion at compile time. In contrast, the [assert Macro and _assert and _wassert functions](../c-runtime-library/reference/assert-macro-assert-wassert.md) test a software assertion at run time and incur a run time cost in space or time. The **static_assert** declaration is especially useful for debugging templates because template arguments can be included in the *constant-expression* parameter.

The compiler examines the **static_assert** declaration for syntax errors when the declaration is encountered. The compiler evaluates the *constant-expression* parameter immediately if it does not depend on a template parameter. Otherwise, the compiler evaluates the *constant-expression* parameter when the template is instantiated. Consequently, the compiler might issue a diagnostic message once when the declaration is encountered, and again when the template is instantiated.

You can use the **static_assert** keyword at namespace, class, or block scope. (The **static_assert** keyword is technically a declaration, even though it does not introduce new name into your program, because it can be used at namespace scope.)

## Description

In the following example, the **static_assert** declaration has namespace scope. Because the compiler knows the size of type `void *`, the expression is evaluated immediately.

## Example

```cpp
static_assert(sizeof(void *) == 4, "64-bit code generation is not supported.");
```

## Description

In the following example, the **static_assert** declaration has class scope. The **static_assert** verifies that a template parameter is a *plain old data* (POD) type. The compiler examines the **static_assert** declaration when it is declared, but does not evaluate the *constant-expression* parameter until the `basic_string` class template is instantiated in `main()`.

## Example

```cpp
#include <type_traits>
#include <iosfwd>
namespace std {
template <class CharT, class Traits = std::char_traits<CharT> >
class basic_string {
    static_assert(std::is_pod<CharT>::value,
                  "Template argument CharT must be a POD type in class template basic_string");
    // ...
    };
}

struct NonPOD {
    NonPOD(const NonPOD &) {}
    virtual ~NonPOD() {}
};

int main()
{
    std::basic_string<char> bs;
}
```

## Description

In the following example, the **static_assert** declaration has block scope. The **static_assert** verifies that the size of the VMPage structure is equal to the virtual memory pagesize of the system.

## Example

```cpp
#include <sys/param.h> // defines PAGESIZE
class VMMClient {
public:
    struct VMPage { // ...
           };
    int check_pagesize() {
    static_assert(sizeof(VMPage) == PAGESIZE,
        "Struct VMPage must be the same size as a system virtual memory page.");
    // ...
    }
// ...
};
```

## See also

[Assertion and User-Supplied Messages (C++)](../cpp/assertion-and-user-supplied-messages-cpp.md)<br/>
[#error Directive (C/C++)](../preprocessor/hash-error-directive-c-cpp.md)<br/>
[assert Macro, _assert, _wassert](../c-runtime-library/reference/assert-macro-assert-wassert.md)<br/>
[Templates](../cpp/templates-cpp.md)<br/>
[ASCII Character Set](../c-language/ascii-character-set.md)<br/>
[Declarations and Definitions](declarations-and-definitions-cpp.md)