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G++ refuses to template class code, ms compiler is happy

0 votes
29 views

I'm not a c++/template guru and i can't decide if this is really a valid code or i have encountered a gcc bug:

template  struct A {
 void *p;
};
template  struct B : A {
 void *foo() { return p; }
};

g++ says "error: 'p' was not declared in this scope". microsoft's compiler is happy with the same code.

Can anyone help?

posted Jun 14, 2013 by anonymous

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1 Answer

+1 vote

Current MSVC in standard C++ mode? That's odd.

The compiler doesn't now the members of A when the B template is compiled. (There might be a partial specialization later in the file.) Therefore, you must explicitly request dependent name lookup by
dereferencing the this pointer:

template  struct B : A {
 void *foo() { return this->p; }
};
answer Jun 14, 2013 by anonymous
Similar Questions
0 votes

This code compiles on MSVC but not on G++ :

template
class B
{
public:
 static void SomeMethod()
 {
 A::SomeMethod();
 }
};

class A
{
public:
 static void SomeMethod();
};

void test()
{
 B::SomeMethod();
}

The error with g++ is :

test.cpp: In static member function "static void B::SomeMethod()":
test.cpp:7:3: error: "A" has not been declared

Is there is a option to indicate to g++ to evaluate the template functions only when a specialization is called, and not before ?

–1 vote

I have a c++ shared object that I am compiling using the following command.
g++ -g -Wall -c -o cpplib.o cpplib.cpp

And I am creating the shared object using the following command.
g++ -shared -o libcpplib.so cpplib.o

(I am aware that I can do this one single command, these are two separate invocations because I had the project setup on eclipse and that's how eclipse cdt does it.).

My host runs a RedHat 6.x x86_64 OS.

The compilation fails with the following error.

$ g++ -g -Wall -fPIC -c -o cpplib.o cpplib.cpp
$ g++ -shared -fPIC -o libcpplib.so cpplib.o
/usr/bin/ld: /usr/lib/gcc/x86_64-redhat-linux/4.4.7/libstdc++.a(ios_init.o): relocation R_X86_64_32 against `pthread_cancel' can not be used when making a shared object; recompile with -fPIC
/usr/lib/gcc/x86_64-redhat-linux/4.4.7/libstdc++.a: could not read symbols: Bad value
collect2: ld returned 1 exit status

I am certain that I haven't used the 'pthread_cancel' function in my code. Please help me.

0 votes

Reading the gcc 5.1 manual it seems that VLA parameters must be accepted by g++. For example the following is fine when compiled with gcc --std=gnu11

float sum (int M, float x[M]) {
 return x[0] + x[1];
}

int main (void) {
 float xs[2] = {1, 2};
 float acc = sum (2, xs);
 return 0;
}

But it is rejected by g++ --std=gnu++11. Is this possible at all with g++?

0 votes

I am new to C+ and I wonder why this code compiles fine with g+ and MSVC:

 namespace t {

 class A {
 public:
 int i;
 };

 int b(int k, A a) {
 return k;
 }

 }

 int main()
 {
 t::A cl;
 return b(5, cl);
 }

I thought that b should not be visible from the main function.

0 votes

I recently encountered a problem with a function was marked as a constructor. The source file was compiled with -O0, but it appears the function was optimized to the point it skipped some of the startup
code and jumped into the failure state (which called exit). I was able to restore desired behavior with '#pragma GCC optimize("O0")' around the function (even volatile tricks did not help).

The startup code had to do with an integrity check. The expected fingerprint was back-patched after compiling, and then recalculated at runtime. Then, a memcmp was made. It appears the compiler deduced that the allocation was a string of 0's and could never be equal to the runtime fingerprint, so its just omitted the code.

Is this expected behavior for functions marked as constructors (compiled with -O0)?


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