Re: pass string by value because of "mean what you say"?

From:
SG <s.gesemann@gmail.com>
Newsgroups:
comp.lang.c++.moderated
Date:
Mon, 23 Feb 2009 17:55:57 CST
Message-ID:
<3b6da4c3-ff92-48e9-bae3-d470c3c0e845@e5g2000vbe.googlegroups.com>
On 23 Feb., 19:25, Frank Birbacher <bloodymir.c...@gmx.net> wrote:

struct Foo
{
        virtual void setName(std::string const&) =0;
        virtual std::string const& getName() const =0;

};
struct SimpleFoo : Foo
{
        void setName(std::string const& newName) { name = newName; }
        std::string const& getName() const { return name; }
private:
        std::string name;

};
struct BrokenFoo : Foo
{
        void setName(std::string const& newName) { namePtr = newName; }
        std::string const& getName() const { return *namePtr; }
private:
        std::string namePtr;

};


I think you meant
           void setName(std::string const& newName) { namePtr =
&newName; }
           std::string* namePtr;

This will certainly break sooner or later, because someone will pass a
temporary variable to setName. This would be avoided with a signature
like "void setName(std::string newName)" because of pass-by-value.
Wouldn't this follow the saying "say what you mean, mean what you say"
more strictly than a reference argument?


Using pass-by-value for newName is not better than pass-by-reference-
to-const in that respect -- except that the compiler may warn about
storing the address of a local variable in some other place (I'm not
sure if any compiler does this).

I'd keep pass-by-reference-to-const for setName and use *return-by-
value*:

   struct GoodFoo : public Foo
   {
           void setName(std::string const& newName) { name = newName; }
           std::string getName() const { return name; }
   private:
           std::string name;
   };

It's reasonable to use pass-by-reference-to-const in setName(). It'll
avoid an unnessecary copy. Using return-by-value has two advantages:
(a) the user can't use a const_cast to mess with your object's
internals and (b) it allows an object to store the name in something
other than a string object.

Cheers!
SG

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