---
layout: stdlib-reference
---
# attribute [mutating]
## Description
Mark a function or a property or subscript accessor as mutating. A mutating function receives the implicit this parameter
as an inout parameter, so that mutations to members access from this argument will be visible to the caller.
## Signature
[mutating]
## Remarks
By default, Slang treats all member functions as non-mutating. For example, consider the following function:
```csharp
struct S
{
int x;
void foo()
{
x = 1; // error: `x` is not an l-value.
}
}
```
The line x=1 will lead to a compile time error because by-default, all member methods in Slang are non-mutating. To
allow foo to modify x, you can use [mutating] to mark the function as such:
```csharp
struct S
{
int x;
[mutating]
void foo()
{
x = 1; // ok
}
}
```
## See also
[nonmutating].