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69 lines
2.8 KiB
Text
69 lines
2.8 KiB
Text
The keyword tt(const) is often used behind the parameter list of member
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functions. This keyword indicates that a member function does not alter the
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data members of its object. Such member functions are called
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emi(const member functions). In the class tt(Person), we see that the
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accessor functions were declared tt(const):
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verb(
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class Person
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{
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public:
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std::string const &name() const;
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std::string const &address() const;
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std::string const &phone() const;
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size_t mass() const;
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};
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)
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The rule of thumb given in section ref(ConstRule) applies here too:
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whichever appears to the em(left) of the keyword tt(const), is not
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altered. With member functions this should be interpreted as `doesn't alter
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its own data'.
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When implementing a const member function the tt(const) attribute must be
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repeated:
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verb(
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string const &Person::name() const
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{
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return d_name;
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}
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)
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The compiler prevents the data members of a class from being modified
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by one of its const member functions. Therefore a statement like
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verb(
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d_name[0] = toupper(static_cast<unsigned char>(d_name[0]));
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)
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results in a compiler error when added to the above function's definition.
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tt(Const) member functions are used to prevent inadvertent data
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modification. Except for constructors and the destructor (cf. chapter
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ref(MEMORY)) only const member functions can be used with (plain, references
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or pointers to) tt(const) objects.
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Const objects are frequently encounterd as tt(const &) parameters of
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functions. Inside such functions only the object's const members may be
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used. Here is an example:
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verb(
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void displayMass(ostream &out, Person const &person)
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{
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out << person.name() << " weighs " << person.mass() << " kg.\n";
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}
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)
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Since tt(person) is defined as a tt(Person const &) the function
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tt(displayMass) cannot call, e.g.,nl()
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tt(person.setMass(75)).
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The tt(const) member function attribute can be used to
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hi(function overloading) overload member functions. When functions are
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overloaded by their tt(const) attribute the compiler uses the member
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function matching most closely the i(const-qualification) of the object:
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itemization(
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it() When the object is a tt(const) object, only tt(const) member
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functions can be used.
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it() When the object is not a tt(const) object, non-tt(const) member
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functions are used, em(unless) only a tt(const) member function is
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available. In that case, the tt(const) member function is used.
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)
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The next example illustrates how (non) tt(const) member functions are
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selected:
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verbinclude(-a examples/constmembers.cc)
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As a general principle of design: member functions should always be given
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the tt(const) attribute, unless they actually modify the object's data.
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