CODEWITHSUNDEEP
c++forward-declarationanonymous-struct
Antonio Di Bacco
Antonio Di Bacco

Reputation: 31

C++: struct forward declaration of anonymous struct causes "conflicting declaration"

I have got this hpp file:

struct rte_spinlock_t;

class A {

    public:
        void init();
    private:
        rte_spinlock_t* spinlock;
};

and the corresponding cpp file:

#include "A.hpp"

typedef struct {
    int lock;
} rte_spinlock_t;

void A::init()
{

}

Now, compiling like this: g++ A.cpp I get this error:

A.cpp:5:3: error: conflicting declaration ‘typedef struct rte_spinlock_t rte_spinlock_t’
    5 | } rte_spinlock_t;
      |   ^~~~~~~~~~~~~~
In file included from A.cpp:1:
A.hpp:2:8: note: previous declaration as ‘struct rte_spinlock_t’
    2 | struct rte_spinlock_t;
      |        ^~~~~~~~~~~~~~

Obviously making the struct named it works, unfortunately I cannot control the typedef of struct rte_spinlock_t that is in a library.

How could I workaround this?

I expect to be able to forward an unnamed struct without getting into conflicting declaration.

Upvotes: 3

Views: 236

Answers (1)

alagner
alagner

Reputation: 4062

From what I understand what you have is more-less this (erroneous code): https://godbolt.org/z/zarsTq6oE

Why don't you use pimpl (private implementation) idiom for extra level of indirection and hiding the "gory details"?

Let's assume my X is your A, lib.h contains the troublesome typedef:

//example.cpp
#include "example.hpp"
#include "lib.h"

struct Impl
{
    CStruct* cs;
};

void X::init()
{
    clib_init(&impl->cs);
}

X::X()
{
    impl = std::make_unique<Impl>();
}
X::~X() = default;

//example.hpp
#pragma once
#include <memory>

struct Impl;

struct X
{
    X();
    ~X();

    //rest of special functions omitted for brevity
    //feel free to add them at your own leisure
    void init();
    std::unique_ptr<Impl> impl;

};

Live demo: https://godbolt.org/z/4jYGrYEjr

BTW, I made an assumption that your troublesome struct is C code based on some searching...

Upvotes: 3

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