Re: RAII object in constructor

James Kanze <>
Thu, 10 Oct 2013 07:49:08 -0700 (PDT)
On Wednesday, 9 October 2013 21:48:50 UTC+1, Marcel M=FCller wrote:

I have a mutex lock that is implemented as RAII. I need to acquire the
mutex as long as the constructor runs. But for the /entire/ constructor=


run not just the construction of the derived class.


#include <stdio.h>

// some mutex class
class Mutex
   // hold the Mutex (RAII)
   struct Lock
   { Lock(Mutex& mtx)
     { puts("Lock()\n");
     { puts("~Lock()\n");

How is this fundamentally different from `std::mutex` and

// set of configuration parameters
struct Parameters
{ //...

class WorkerBase
{ //...
   WorkerBase(const Parameters& params)
   { // do something with params...

class Worker : public WorkerBase
   static Parameters GlobalParams;
   static Mutex ParamMtx; // protect the above

   // synchronized public access to the parameters
   struct AccessParams : Mutex::Lock
   { AccessParams() : Mutex::Lock(ParamMtx) {}
     operator Parameters&() { return GlobalParams; }

   Worker() : WorkerBase(AccessParams())
   { AccessParams params; // !!! Mutex acquired twice!
     // do something with params...

The simplest solution (that I know) is to make the constructor
to Worker take an `AccessParams` argument:

    Worker::Worker( AccessParams const& params = AccessParams() )
        : WorkerBase( params )

There are a number of variants on this, depending on whether
`Worker` requires more arguments or not, but the basic idea is
for the most derived class' constructor to have the special

Note that if you ever use `Worker` as a temporary, the lock will
be held until the end of the full expression. This can be a bit
of a bother, if e.g. client code does something like:

    func( Worker() );

The lock will be held during the entire call to func, which
could be a problem.


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