---
title: "__thiscall"
ms.date: "11/04/2016"
f1_keywords: ["__thiscall", "__thiscall_cpp"]
helpviewer_keywords: ["__thiscall keyword [C++]"]
ms.assetid: a6a22dd2-0101-4885-b33b-22f6057965df
---
# __thiscall

**Microsoft Specific**

The **__thiscall** calling convention is used on member functions and is the default calling convention used by C++ member functions that do not use variable arguments. Under **__thiscall**, the callee cleans the stack, which is impossible for `vararg` functions. Arguments are pushed on the stack from right to left, with the **this** pointer being passed via register ECX, and not on the stack, on the x86 architecture.

One reason to use **__thiscall** is in classes whose member functions use `__clrcall` by default. In that case, you can use **__thiscall** to make individual member functions callable from native code.

When compiling with [/clr:pure](../build/reference/clr-common-language-runtime-compilation.md), all functions and function pointers are `__clrcall` unless specified otherwise. The **/clr:pure** and **/clr:safe** compiler options are deprecated in Visual Studio 2015 and unsupported in Visual Studio 2017.

In releases before Visual Studio 2005, the **__thiscall** calling convention could not be explicitly specified in a program, because **__thiscall** was not a keyword.

`vararg` member functions use the **__cdecl** calling convention. All function arguments are pushed on the stack, with the **this** pointer placed on the stack last

Because this calling convention applies only to C++, there is no C name decoration scheme.

On ARM and x64 machines, **__thiscall** is accepted and ignored by the compiler.

For non-static class functions, if the function is defined out-of-line, the calling convention modifier does not have to be specified on the out-of-line definition. That is, for class non-static member methods, the calling convention specified during declaration is assumed at the point of definition.

**END Microsoft Specific**

## See also

[Argument Passing and Naming Conventions](../cpp/argument-passing-and-naming-conventions.md)