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Thinking in C++
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Scoping rules tell you where a variable is valid, where it is created, and where it gets destroyed (i.e., goes out of scope). The scope of a variable extends from the point where it is defined to the first closing brace that matches the closest opening brace before the variable was defined. That is, a scope is defined by its “nearest” set of braces. To illustrate:

//: C03:Scope.cpp
// How variables are scoped
int main() {
  int scp1;
  // scp1 visible here
    // scp1 still visible here
    int scp2;
    // scp2 visible here
      // scp1 & scp2 still visible here
      int scp3;
      // scp1, scp2 & scp3 visible here
      // ...
    } // <-- scp3 destroyed here
    // scp3 not available here
    // scp1 & scp2 still visible here
    // ...
  } // <-- scp2 destroyed here
  // scp3 & scp2 not available here
  // scp1 still visible here
} // <-- scp1 destroyed here

The example above shows when variables are visible and when they are unavailable (that is, when they go out of scope). A variable can be used only when inside its scope. Scopes can be nested, indicated by matched pairs of braces inside other matched pairs of braces. Nesting means that you can access a variable in a scope that encloses the scope you are in. In the example above, the variable scp1 is available inside all of the other scopes, while scp3 is available only in the innermost scope.

Thinking in C++
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   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire