为什么在非指针结构上进行类型转换会出现语法错误

Question

I am using Visual C++ express 2008 try to compile code similar to below:

no problem

{
  ...
  AVRational test = {1, 1000};
  ...
}

but has problem when it is as below:

{
  ...
  AVRational test = (AVRational){1, 1000};
  ...
}

gave errors:

1>..\..\..\projects\test\xyz.cpp(1139) : error C2059: syntax error : '{'
1>..\..\..\projects\test\xyz.cpp(1139) : error C2143: syntax error : missing   ';' before '{'
1>..\..\..\projects\test\xyz.cpp(1139) : error C2143: syntax error : missing ';' before '}'

where AVRational (ffmpeg.org library) is defined as:

typedef struct AVRational{
    int num; ///< numerator
    int den; ///< denominator
} AVRational;

FFmpeg come with some pre-define value such as

#define AV_TIME_BASE_Q (AVRational){1, AV_TIME_BASE}

which is used as below

av_rescale_q(seek_target, AV_TIME_BASE_Q, pFormatCtx->streams[stream_index]->time_base);

will failed to compile on Visual C++ express 2008

It seem like the same code will be compiled with no error/warning on gcc compiler. Why I get this error on VC++? Is it a C/C++ standard way to do casting on struct value? Anyway I can avoid this error while still able to use the defined AV_TIME_BASE_Q?

Answer 1

Use av_get_time_base_q() instead of AV_TIME_BASE_Q for C++ or VS. This was fixed in a patch

Answer 2

VC++ 2013 does not allow compound literals in C++ but it allows them in C. Options:

1. Rename your program with a .c suffix
2. Switch on the /TC flag for the program that does not compile.

The other alternative if you wish to keep to C++ is to change the declaration of AV_TIME_BASE_Q in the header file

static const AVRational AV_TIME_BASE_Q = {1, AV_TIME_BASE};

Then it will be using the constant instead of the compound literal.

Answer 3

For compound-literals errors in C++

wrong:

 this->buffer.enqueue((tone_t) { duration, frequency });

correct:

tone_t tone = { duration, frequency };
this->buffer.enqueue(tone);