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## Struct by_value_generator

boost::proto::by_value_generator — A generator that replaces child nodes held by reference with ones held by value. Use with `proto::compose_generators<>` to forward that result to another generator.

## Synopsis

    // In header: <boost/proto/generate.hpp>

    struct by_value_generator :  proto::callable {
      template<typename This, typename Expr>
      struct result<This(Expr)> {
        // types
        typedef unspecified type;
      };

      // public member functions
      template<typename Expr> unspecified operator()(Expr const &) const;
    };

## Description

Generators are intended for use as the first template parameter to the `proto::domain<>` class template and control if and how expressions within that domain are to be customized. `proto::by_value_generator` ensures all child nodes are held by value. This generator is typically composed with a second generator for further processing, as `proto::compose_generators<proto::by_value_generator, MyGenerator>`.

### `by_value_generator` public member functions

1.     template<typename Expr> unspecified operator()(Expr const & expr) const;

   Parameters: |
   `expr` | A Proto expression.
   Returns: | Equivalent to `proto::deep_copy(expr)`