---
title: "__thiscall"
description: "Learn about the Microsoft-specific __thiscall calling convention for x86 class member functions in Microsoft C++."
ms.date: "05/22/2020"
f1_keywords: ["__thiscall", "__thiscall_cpp"]
helpviewer_keywords: ["__thiscall keyword [C++]"]
ms.assetid: a6a22dd2-0101-4885-b33b-22f6057965df
---
# `__thiscall`

The **Microsoft-specific** **`__thiscall`** calling convention is used on C++ class member functions on the x86 architecture. It's the default calling convention used by member functions that don't use variable arguments (`vararg` functions).

Under **`__thiscall`**, the callee cleans the stack, which is impossible for `vararg` functions. Arguments are pushed on the stack from right to left. The **`this`** pointer is passed via register ECX, and not on the stack.

On ARM, ARM64, and x64 machines, **`__thiscall`** is accepted and ignored by the compiler. That's because they use a register-based calling convention by default.

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.

`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++, it doesn't have a C name decoration scheme.

When you define a non-static class member function out-of-line, specify the calling convention modifier only in the declaration. You don't have to specify it again on the out-of-line definition. The compiler uses the calling convention specified during declaration at the point of definition.

## See also

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