Re: typedef float float4[4]; std::vector<float4>; does not compile, why?

From:

Brian Cole <coleb2@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Tue, 19 May 2009 10:55:03 -0700 (PDT)

Message-ID:

<6ba5517a-df11-406f-af4d-f3e4cce7e23b@c18g2000prh.googlegroups.com>

On May 19, 11:18 am, Jeff Schwab <j...@schwabcenter.com> wrote:

Brian Cole wrote:

On May 19, 10:34 am, Jeff Schwab <j...@schwabcenter.com> wrote:

Brian Cole wrote:

On May 19, 2:01 am, SG <s.gesem...@gmail.com> wrote:

On 19 Mai, 00:12, Brian Cole <col...@gmail.com> wrote:

The following code will not compile in gcc 4.3.2 on Ubuntu 8.10
#include <vector>
typedef float float4[4];
int main()
{
std::vector<float4> vals;
}

As Alf said, float4 doesn't satisfy the vector templates' requiremen=

as a value type.
If your C++ implementation ships with the TR1 library extension or y=

can install some TR1 implementation and/or Boost you could solve thi=

problem via:
typedef std::tr1::array<float,4> float4;

Apparently GCC considers this a bug (http://gcc.gnu.org/bugzilla/
show_bug.cgi?id=40192). So now I'm doubly confused.

It wasn't a bug. They should not have "fixed" it.

I'm not a language lawyer, is "float4" a pointer to an array of 4
floats? Or is it an array of 4 floats?

It's an array (in your original post). C++ raw arrays decay to
pointers, but that's not something you should need to worry about unle=

you're manually laying out memory. You almost never should need a r=

array. Forget about raw arrays.

Basically if we define float4_t
as "struct float4_t { float x,y,z,w; };" is std::vector<float4>
equivalent to std::vector<float4_t> or std::vector<float4_t *>.

Neither. std::vector<float[4]> shouldn't even be legal.

FYI, the reason this arose is because I am toying with the OpenCL
libraries which defines the type:http://www.khronos.org/registry/cl/api=

/1.0/cl_platform.h

If it truly is illegal I think OpenCL should change the definition to
a struct to be friendlier with C++.

Khronos probably should have defined:

struct float4 { float values[4]; }

In C, though, it is common to use arrays directly as containers, for
historical reasons, and OpenCL is apparently based on C99. OpenCL is
full of other C-related nastiness, too; for example, it defines names
like float4 directly in the global namespace. Namespace pollution is a
real problem in large programs.

One easy way around the vector<float4> problem is to wrap float4 in your
own class.

There is a C++ wrapper layer being developed on top of OpenCL:
http://www.khronos.org/registry/cl/api/1.0/cl.hpp It will be part of
the 1.1 standard.

This is perhaps a nastiness that should be dealt with there now that I
am thoroughly convinced that cl_float4 does not play nice with C++.

-Brian