Static structure initialization with tags in C++

问题:

I've searched stackoverflow for an answer but I cannot get something relevant.

I'm trying to initialize a static structure instance with initial values by specifying their tags, but I get an error at compilation time:

src/version.cpp:10: error: expected primary-expression before ‘.’ token

Here's the code:

// h
typedef struct
{
    int lots_of_ints;
    /* ... lots of other members */
    const char *build_date;
    const char *build_version;
} infos;

And the faulty code:

// C

static const char *version_date = VERSION_DATE;
static const char *version_rev  = VERSION_REVISION;

static const infos s_infos =
{
   .build_date    = version_date, // why is this wrong? it works in C!
   .build_version = version_rev
};

const infos *get_info()
{
    return &s_infos;
}

So the basic idea is to bypass the "other members" initialization and only set the relevant build_date and build_version values. This used to work in C, but I can't figure out why it won't work in C++.

Any ideas?

edit:

I realize this code looks like simple C, and it actually is. The whole project is in C++ so I have to use C++ file extensions to prevent the makefile dependency mess ( %.o: %.cpp )

回答1:

The feature you are using is a C99 feature and you are using a C++ compiler that doesn't support it. Remember that although C code is generally valid C++ code, C99 code isn't always.

回答2:

The following example code defines a struct in what I consider a more C++ way (no need for typedef) and uses a constructor to solve your problem:

#include <iostream>

#define VERSION_DATE "TODAY"
#define VERSION_REVISION "0.0.1a"

struct infos {
    int lots_of_ints;
    /* ... lots of other members */
    const char *build_date;
    const char *build_version;

    infos() : 
        build_date(VERSION_DATE), 
        build_version(VERSION_REVISION) 
    {} 
};

static const infos s_infos;

const infos *get_info()
{
    return &s_infos;
}

int main() {

    std::cout << get_info()->build_date << std::endl;
    std::cout << get_info()->build_version << std::endl;

    return 0;
}

回答3:

I believe this was added as a feature in C99, but has never been a standard feature in C++.

However, some compilers probably offer it as a non-standard language extension.