C++ error: expected primary-expression before ‘.’ token

Question

I looked at the earlier questions but still i was not satisfied, hence i am posting this. I was trying to compile the C++ code written by someone else.

/*
file1.h
*/
#include <stdio.h>
#include <stdlib.h>
typedef struct
{
    struct
    {   
        unsigned member1;
        unsigned  member2; 
    } str1;

    struct
    {
        unsigned member3;
        unsigned  member4; 
    } str2;

    struct
    {
        unsigned member5;
        unsigned  member6; 
    } str3;
} CONFIG_T;



/* 
file1.c
*/
CONFIG_T  cfg =
{
    .str1 = { 0x01, 0x02 },
    .str2 = { 0x03, 0x04 },
    .str3 = { 0x05, 0x06 }
};

Compiled with std C++11 and i get below error. Why the '.' has been used in code while assigning values ?

home $$  g++ -c -std=gnu++0x  initialze_list.cpp

initialze_list.cpp:34: error: expected primary-expression before ‘.’ token

initialze_list.cpp:35: error: expected primary-expression before ‘.’ token

initialze_list.cpp:36: error: expected primary-expression before ‘.’ token

I was not able to understand the reason for error. Please help.

Solution 1

What you posted is C code, not C++ code (note the .c file extention). However, the following code:

CONFIG_T  cfg =
{
    { 0x01, 0x02 },
    { 0x03, 0x04 },
    { 0x05, 0x06 }
};

should work fine.

You can also read about C++11 initialize lists in the wiki.

Solution 2

Designated aggregate initializers is a C99 feature, i.e. it is a feature of C language. It is not present in C++.

If you insist on compiling it as C++, you'll have to rewrite the initialization of cfg.

Solution 3

/* 
file1.c
*/
CONFIG_T  cfg =
{
  .str1 = { 0x01, 0x02 },
  .str2 = { 0x03, 0x04 },
  .str3 = { 0x05, 0x06 }
};

That code is using a C99 feature called designated initializers. As you have observed, that feature is not available in C++ and C++11.

---

As suggested in this answer you should use a C compiler for C code. You can still link it to your C++ application. You could use cmake to do the build configuration for you. A simple example:

/* f1.h */
#ifndef MYHEADER
#define MYHEADER

typedef struct { int i, j; } test_t; 
extern test_t t;

#endif

---

/* f1.c */
#include "f1.h"
test_t t = { .i = 5, .j = 6 };

---

/* f0.cc */
extern "C" { #include "f1.h" }
#include <iostream>

int main() {
    std::cout << t.i << " " << t.j << std::endl;
}

---

# CMakeLists.txt
add_executable(my_executable f0.cc f1.c)

just run mkdir build; cd build; cmake ..; make from your source directory.