According to ISO/IEC 14882 (third edition 2011-09-01) : {cpp}11


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The register specifier shall be applied only to names of variables declared in a block or to function parameters. It specifies that the named variable has automatic storage duration. A variable declared without a storage-class-specifier at block scope or declared as a function parameter has automatic storage duration by default.

A register specifier is a hint to the implementation that the variable so declared will be heavily used.[ Note: The hint can be ignored and in most implementations it will be ignored if the address of the variable is taken. This use is *deprecated* - end note ]

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In upcoming versions of C/{cpp} we can expect this deprecated specifier to not be supported anymore. 

