---
title: Warning C26451
description: "Describes the causes of MSVC code analysis warning C26451, and shows how to fix it."
ms.date: 07/15/2020
f1_keywords: ["C26451", "RESULT_OF_ARITHMETIC_OPERATION_CAST_TO_LARGER_SIZE"]
helpviewer_keywords: ["C26451"]
---
# Warning C26451

> Arithmetic overflow: Using operator '*operator*' on a *size-a* byte value and then casting the result to a *size-b* byte value. Cast the value to the wider type before calling operator '*operator*' to avoid overflow (io.2)

This warning indicates incorrect behavior that results from integral promotion rules and types larger than the ones in which arithmetic is typically performed.

## Remarks

Code analysis detects when an integral value gets shifted left, multiplied, added, or subtracted, and the result gets cast to a wider integral type. If the operation overflows the narrower integral type, then data is lost. You can prevent this loss by casting the value to a wider type before the arithmetic operation.

Code analysis name: `RESULT_OF_ARITHMETIC_OPERATION_CAST_TO_LARGER_SIZE`

## Example 1

The following code generates this warning:

```cpp
void leftshift(int i) noexcept
{
  unsigned __int64 x;
  x = i << 31;  // C26451 reported here

  // code
}
```

To correct this warning, use the following code:

```cpp
void leftshift(int i) noexcept
{
  unsigned __int64 x;
  x = static_cast<unsigned __int64>(i) << 31; // OK

  // code
}
```

## Example 2

```cpp
void somefunc(__int64 /* y */) noexcept
{}

void callsomefunc(int x) noexcept
{
  somefunc(x * 2); // C26451 reported here
}
```

To correct this warning, use the following code:

```cpp
void callsomefunc(int x) noexcept
{
  somefunc(static_cast<__int64>(x) * 2); // OK
}
```

## Example 3

```cpp
__int64 add(int x) noexcept
{
  constexpr auto value = 2;
  return x += value; // C26451 reported here
}
```

To correct this warning, use the following code:

```cpp
__int64 add(int x) noexcept
{
  constexpr auto value = 2;
  const __int64 y = static_cast<__int64>(x) + value; // OK
  return y;
}
```

## See also

- [ES.103: Don't overflow](https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#Res-overflow)