C to C++ table inline definition

Question

I have code in C that compiles and works correctly and I would like to use similar code in C++:

static const char* aTable[12] = {
    [4]="seems",
    [6]=" it  ",
[8]="works",};

int main(){
    printf("%s%s%s", aTable[4],aTable[6],aTable[8]); 
    return 0;
}

Now if I put it in a .c file and compiles with gcc it works. But, if I put it in a .cpp file and compile it with g++, I get the following errors:

test_cpp.cpp:5:3: error: expected identifier before numeric constant
test_cpp.cpp:5:4: error: type '<lambda>' with no linkage used to declare function 'void<lambda>::operator()() const' with linkage [-fpermissive] 
test_cpp.cpp: In lambda function: test_cpp.cpp:5:5: error: expected '{' before '=' token 
test_cpp.cpp: At global scope: test_cpp.cpp:5:5: warning: lambda expressions only available with
    -std=c++0x or -std=gnu++0x [enabled by default] 
test_cpp.cpp:5:6: error: no match for 'operator=' in '{} = "seems"' test_cpp.cpp:5:6: note: candidate is: test_cpp.cpp:5:4: note: <lambda()>&<lambda()>::operator=(const<lambda()>&) 
test_cpp.cpp:5:4: note:   no known conversion for argument 1 from 'const char [6]' to 'const<lambda()>&' 
test_cpp.cpp:6:3: error: expected identifier before numeric constant
test_cpp.cpp:6:4: error: type '<lambda>' with no linkage used to declare function 'void<lambda>::operator()() const' with linkage [-fpermissive]

Is there a way to express that I am not declaring a lambda function, just trying to fill a table?

I would like to keep the following part :

[4]="seems",
[6]=" it  ",
[8]="works",

because it comes from an autogenerated file...

Remember that C and C++ are different languages? Even for historical version of compilers there was some incompatibilities, then for modern extensions of recent compilers problem is worst. — Jean-Baptiste Yunès, May 29 '18 at 11:16
These are called "designated initializers" - helps doing google searches. Though I didn't find anything good. — Antti Haapala -- Слава Україні, May 29 '18 at 11:16
There is no comparable syntax, you just have to put them in the correct order and fill in the blanks. — Galik, May 29 '18 at 11:16
its quite rare that both the c and c++ tags are ontopic. On top of that this nicely demonstrates that c++, in contrast to common misbelieve, is not a superset of c. What a nice question :) — 463035818_is_not_an_ai, May 29 '18 at 11:21
@aschepler I thought the C++20 proposal was only for `.name = value`, not `[index] = value`. — , May 29 '18 at 12:31
Doesn't GCC support designated initialisers as *extension*? Would require to compile with one of the `-std=gnuXYZ` modes, wouldn't it? — Aconcagua, May 29 '18 at 12:47

SHR · Accepted Answer · 2018-05-30T07:56:14.607

28

You can mix C and C++ code easily.

You should keep the C code to be compiled with C compiler (gcc), rest of the code can be C++ and be compiled with C++ compiler (g++). then link all object (.o) files together.

like this:

file name: a.c

const char* aTable[12] = {
    [4]="seems",
    [6]=" it  ",
[8]="works",};

file name: b.cpp

#include <cstdio>
extern "C" const char* aTable[12];   
int main(){
    printf("%s%s%s", aTable[4],aTable[6],aTable[8]); 
    return 0;
}

Now compile:

gcc -c a.c -o a.o
g++ -c b.cpp -o b.o
g++ b.o a.o -o all.out

Now run the executable (all.out) and you'll see that everything will work.

Just note that for functions you'll need to add extern "C" before the declaration in the cpp file.

edited May 30 '18 at 07:56

answered May 29 '18 at 12:16

SHR

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That should be `extern "C" const char* aTable[12];` too: IIRC at least one compiler performs name mangling otherwise. – May 29 '18 at 12:27
@hvd In g++\gcc (both was tagged in the question) it works without the `extern "C"`, And I tried to make it as simple as possible. – SHR May 29 '18 at 15:04
1

Bonus: As the table is generated, it certainly is a file of its own anyway – Hagen von Eitzen May 29 '18 at 17:57
6

@SHR It is better to keep it as correct as possible while remaining simple. – Yakk - Adam Nevraumont May 29 '18 at 19:55
Ok, I've added the `extern "C"`. – SHR May 30 '18 at 07:51

score 16 · Answer 2 · answered May 29 '18 at 11:17

There's no way to do this. C++ never adopted this particular C syntax.

In your case, since the table is auto-generated, I would just put it in a C file. As long as it's not marked static, it can be accessed from C++ code without problems.

score 13 · Answer 3 · answered May 29 '18 at 11:56

13

[4]= is a designated initializer, one of the C features not supported by C++.

Here is a list that tries to list which C99 features that are supported by C++17. Scroll down to the bottom to see which ones that are not supported.

It is important to realize that C++ is by no means a superset of C.

answered May 29 '18 at 11:56

Lundin

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C and C++ are two languages separated by a common syntax. – Eljay May 29 '18 at 12:01

score 5 · Answer 4 · answered May 29 '18 at 12:53

Designated initialisers not being supported by C++ already mentioned – if you insist on C++, you could write a wrapper class with a constructor instead:

class Table
{
    std::array<char const*, 12> data;
public:
    Table()
    {
        data[4] = "seems";
        data[6] = " it  ";
        data[8] = "works";
    }
    char const* operator[](unsigned int index) const
    {
        return data[index];
    }
} aTable;

Did not test the code, so if you find a bug, feel free to fix it yourself...

ivaigult · Answer 5 · 2018-05-29T14:52:37.537

As said above, this type of initialization is not supported in C++, but you could use lambda function to initialize static array<const char*, 12> like this:

#include <array>
#include <cstdio>

static const std::array<const char*, 6> aTable = []() {
    std::array<const char*, 6> table;
    table[0] = "Hello";
    table[3] = ", ";
    table[5] = "world!";
    return std::move(table);
}();

int main() {
    printf("%s%s%s\n", aTable[0], aTable[3], aTable[5]);
    return 0;
}

Demo on coliru

Notable, that compiler will do RVO here, and the initialization will be performed in-place. This is assembly generated by g++-5.4.0:

<_GLOBAL__sub_I_main>:
movq   $0x40064c,0x200bc5(%rip)        # 601060 <_ZL6aTable>
movq   $0x400652,0x200bd2(%rip)        # 601078 <_ZL6aTable+0x18>
movq   $0x400655,0x200bd7(%rip)        # 601088 <_ZL6aTable+0x28>
retq

C to C++ table inline definition

5 Answers5