"expected constructor, destructor, or type conversion before '->' token" using a Singleton

[Damien]

Hi all,

I'm using a pretty standard C++ Singleton class, as below:

template <typename T>
class Singleton
{
public:
static T* Instance()
{
static T instance_;
return &instance_;
}

private:
Singleton(); // ctor hidden
~Singleton(); // dtor hidden
Singleton(Singleton const&); // copy ctor hidden
Singleton& operator=(Singleton const&); // assign op hidden
};

Trying to use it on something really simple is giving me headaches:

class DoIt
{
public:
DoIt(){}
~DoIt(){}

void DoSomething(){}
};

int main()
{
DoIt* doit = Singleton<DoIt>::Instance();
doit->DoSomething();
}

I get the compiler error "error: expected constructor, destructor, or
type conversion before '->' token" on the doit->DoSomething(); line. I
can't see how doit can be name-dependent because there's no typedef'ing
associated with it inside the Singleton, unless the static has
something to do with it. Anyone got any ideas?

Damien

 

[Ivan Vecerina]

: I'm using a pretty standard C++ Singleton class, as below:
:
: template <typename T>
: class Singleton
: {
: public:
: static T* Instance()
: {
: static T instance_;
: return &instance_;
: }
:
: private:
: Singleton(); // ctor hidden
: ~Singleton(); // dtor hidden
: Singleton(Singleton const&); // copy ctor hidden
: Singleton& operator=(Singleton const&); // assign op hidden
: };
:
: Trying to use it on something really simple is giving me headaches:
:
: class DoIt
: {
: public:
: DoIt(){}
: ~DoIt(){}
:
: void DoSomething(){}
: };
:
: int main()
: {
: DoIt* doit = Singleton<DoIt>::Instance();
: doit->DoSomething();
: }
:
: I get the compiler error "error: expected constructor, destructor, or
: type conversion before '->' token" on the doit->DoSomething(); line.
I
: can't see how doit can be name-dependent because there's no
typedef'ing
: associated with it inside the Singleton, unless the static has
: something to do with it. Anyone got any ideas?
Well, the code you posted compiles "as is" in both Comeau and VC8.1.
However, the design of your singleton seems weird to me (was it an
artificial example posted for demonstration purposes?), and it does
not prevent the creation of additional DoIt instances.

Your "Singleton" class could be replaced with a single function:
template<class T>
T& single_instance() { static T instance; return instance; }

Usage:
int main()
{
DoIt& doit = single_instance<DoIt>();
doit.DoSomething();
}

An online search will probably bring a few existing singleton
implementations, up to Andrei Alexandrescu's policy-based design.
But if the 'static function member' does what you need, I personally
wouldn't bother using a Singleton template to implement it.

hth -Ivan

 

[eriwik]

Hi all,

I'm using a pretty standard C++ Singleton class, as below:

template <typename T>
class Singleton
{
public:
static T* Instance()
{
static T instance_;
return &instance_;
}

private:
Singleton(); // ctor hidden
~Singleton(); // dtor hidden
Singleton(Singleton const&); // copy ctor hidden
Singleton& operator=(Singleton const&); // assign op hidden
};

Trying to use it on something really simple is giving me headaches:

class DoIt
{
public:
DoIt(){}
~DoIt(){}

void DoSomething(){}
};

int main()
{
DoIt* doit = Singleton<DoIt>::Instance();
doit->DoSomething();
}

I get the compiler error "error: expected constructor, destructor, or
type conversion before '->' token" on the doit->DoSomething(); line. I
can't see how doit can be name-dependent because there's no typedef'ing
associated with it inside the Singleton, unless the static has
something to do with it. Anyone got any ideas?

You sure this is the code you are trying to compile? It works fine for
me both in VC8 and Comeau Online.

 

[Damien]

I should have said the compiler is gcc 4.1.1. I know it's not a
perfect Singleton design, it's the error message I want to understand.

DAmien

 

[Lionel B]

On Tue, 12 Dec 2006 06:11:22 -0800, Damien wrote:

(Please don't top-post)

I should have said the compiler is gcc 4.1.1. I know it's not a perfect
Singleton design, it's the error message I want to understand.

Compiles ok for me here on gcc 4.1.1 (linux x86_64). Sure there's not
something you're not telling us?

 

[Damien]

Argh. My bad. I'm using SCons and I didn't update something in a
build file, so it wasn't compiling my code above.

The code above is fine. What was broken was a pair of macros
(%$#$#*&!!!) that put an equivalent of doit.DoSomething() outside the
flow of control, so of course it's a declaration and the compiler
correctly complains.

Thank you all and sorry to waste your time,

Damien