Commit aa360a9

ms.date: "11/04/2016"

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**Assign** does a binary copy, which means the entire length of the `BSTR` is copied, regardless of content.

[_bstr_t Class](../cpp/bstr-t-class.md)

ms.date: "11/04/2016"

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See [_bstr_t::Assign](../cpp/bstr-t-assign.md) for an example using **Attach**.

[_bstr_t Class](../cpp/bstr-t-class.md)

ms.date: "11/04/2016"

ms.assetid: 116d994e-5a72-4351-afbe-866c80b4c165

*var*<br/>

*bstr*<br/>

*fCopy*<br/>

If **`false`**, the *bstr* argument is attached to the new object without making a copy by calling `SysAllocString`.

The following table describes the `_bstr_t` constructors.

|Constructor|Description|

|-----------------|-----------------|

|`_bstr_t( )`|Constructs a default `_bstr_t` object that encapsulates a null `BSTR` object.|

|`_bstr_t( _bstr_t&` `s1` `)`|Constructs a `_bstr_t` object as a copy of another.<br /><br /> This is a *shallow* copy, which increments the reference count of the encapsulated `BSTR` object instead of creating a new one.|

|`_bstr_t( char*` `s2` `)`|Constructs a `_bstr_t` object by calling `SysAllocString` to create a new `BSTR` object and then encapsulates it.<br /><br /> This constructor first performs a multibyte to Unicode conversion.|

|`_bstr_t( wchar_t*` `s3` `)`|Constructs a `_bstr_t` object by calling `SysAllocString` to create a new `BSTR` object and then encapsulates it.|

|`_bstr_t( _variant_t&` `var` `)`|Constructs a `_bstr_t` object from a `_variant_t` object by first retrieving a `BSTR` object from the encapsulated VARIANT object.|

|`_bstr_t( BSTR` `bstr` `, bool` `fCopy` `)`|Constructs a `_bstr_t` object from an existing `BSTR` (as opposed to a `wchar_t*` string). If *fCopy* is false, the supplied `BSTR` is attached to the new object without making a new copy with `SysAllocString`.<br /><br /> This constructor is used by wrapper functions in the type library headers to encapsulate and take ownership of a `BSTR` that is returned by an interface method.|

[_bstr_t Class](../cpp/bstr-t-class.md)<br/>

[_variant_t Class](../cpp/variant-t-class.md)

description: "Learn more about: _bstr_t Class"

title: "_bstr_t Class"

ms.date: "11/04/2016"

ms.assetid: 58841fef-fe21-4a84-aab9-780262b5201f

A `_bstr_t` object encapsulates the [BSTR data type](/previous-versions/windows/desktop/automat/bstr). The class manages resource allocation and deallocation through function calls to `SysAllocString` and `SysFreeString` and other `BSTR` APIs when appropriate. The **_bstr_t** class uses reference counting to avoid excessive overhead.

| [_bstr_t](../cpp/bstr-t-bstr-t.md) | Constructs a `_bstr_t` object. |

| [Assign](../cpp/bstr-t-assign.md) | Copies a `BSTR` into the `BSTR` wrapped by a `_bstr_t`. |

| [Attach](../cpp/bstr-t-attach.md) | Links a `_bstr_t` wrapper to a `BSTR`. |

| [copy](../cpp/bstr-t-copy.md) | Constructs a copy of the encapsulated `BSTR`. |

| [Detach](../cpp/bstr-t-detach.md) | Returns the `BSTR` wrapped by a `_bstr_t` and detaches the `BSTR` from the `_bstr_t`. |

| [GetAddress](../cpp/bstr-t-getaddress.md) | Points to the `BSTR` wrapped by a `_bstr_t`. |

| [GetBSTR](../cpp/bstr-t-getbstr.md) | Points to the beginning of the `BSTR` wrapped by the `_bstr_t`. |

| [length](../cpp/bstr-t-length.md) | Returns the number of characters in the `_bstr_t`. |

| [operator =](../cpp/bstr-t-operator-equal.md) | Assigns a new value to an existing `_bstr_t` object. |

| [operator +=](../cpp/bstr-t-operator-add-equal-plus.md) | Appends characters to the end of the `_bstr_t` object. |

| [operator +](../cpp/bstr-t-operator-add-equal-plus.md) | Concatenates two strings. |

| [operator !](../cpp/bstr-t-operator-logical-not.md) | Checks if the encapsulated `BSTR` is a NULL string. |

| [operator ==, !=, \<, >, \<=, >=](../cpp/bstr-t-relational-operators.md) | Compares two `_bstr_t` objects. |

| [operator wchar_t* &#124; char\*](../cpp/bstr-t-wchar-t-star-bstr-t-char-star.md) | Extract the pointers to the encapsulated Unicode or multibyte `BSTR` object. |

**Lib:** comsuppw.lib or comsuppwd.lib (see [/Zc:wchar_t (wchar_t Is Native Type)](../build/reference/zc-wchar-t-wchar-t-is-native-type.md) for more information)

[Compiler COM Support Classes](../cpp/compiler-com-support-classes.md)

ms.date: "11/04/2016"

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*fCopy*<br/>

If **`true`**, **copy** returns a copy of the contained `BSTR`, otherwise **copy** returns the actual BSTR.

Returns a newly allocated copy of the encapsulated `BSTR` object.

ms.date: "11/04/2016"

ms.assetid: cc8284bd-f68b-4fff-b2e6-ce8354dabf8b

The `BSTR` wrapped by the `_bstr_t`.

See [_bstr_t::Assign](../cpp/bstr-t-assign.md) for a example using **Detach**.

[_bstr_t Class](../cpp/bstr-t-class.md)

ms.date: "11/04/2016"

ms.assetid: 09bc9180-867e-4ee5-b22a-8339dc663142

**GetAddress** affects all `_bstr_t` objects that share a `BSTR`. More than one `_bstr_t` can share a `BSTR` through the use of the copy constructor and **operator=**.

See [_bstr_t::Assign](../cpp/bstr-t-assign.md) for a example using **GetAddress**.

[_bstr_t Class](../cpp/bstr-t-class.md)