[C/CPP系列知识] 那些程序C语言可以编译通过但C++无法编译成功 Write a C program that won’t compile in C++

http://www.geeksforgeeks.org/write-c-program-wont-compiler-c/

1) C++中在函数声明之前调用一个函数会引发错误，但是在C中有可能可以。 参考 /article/6746140.html

下面的程序可以用gcc编译，但g++无法编译。

#include<stdio.h>
int main()
{
foo(); // foo() is called before its declaration/definition
}

int foo()
{
printf("Hello");
return 0;
}

编译结果：

diego@ubuntu:~/myProg/geeks4geeks/cpp$ gcc test3.c
diego@ubuntu:~/myProg/geeks4geeks/cpp$ g++ test3.c
test3.c: In function 'int main()':
test3.c:4:9: error: 'foo' was not declared in this scope
foo(); // foo() is called before its declaration/definition

2) C++中将一个非const指针指向一个const变量是非法的，但在C中是可以的。

#include <stdio.h>

int main(void)
{
int const j = 20;

/* The below assignment is invalid in C++, results in error
In C, the compiler *may* throw a warning, but casting is
implicitly allowed */
int *ptr = &j;  // A normal pointer points to const

printf("*ptr: %d\n", *ptr);

return 0;
}

编译结果：

diego@ubuntu:~/myProg/geeks4geeks/cpp$ gcc test4.c
test4.c: In function 'main':
test4.c:10:16: warning: initialization discards 'const' qualifier from pointer target type [enabled by default]
int *ptr = &j;  // A normal pointer points to const
^
diego@ubuntu:~/myProg/geeks4geeks/cpp$ g++ test4.c
test4.c: In function 'int main()':
test4.c:10:17: error: invalid conversion from 'const int*' to 'int*' [-fpermissive]
int *ptr = &j;  // A normal pointer points to const
　　             ^

3) C中可以将void* 赋值给其他的指针，如int*， char*等，但是在C++中则需要显示的类型转换。

#include <stdio.h>
int main()
{
void *vptr;
int *iptr = vptr; //In C++, it must be replaced with int *iptr=(int *)vptr;
return 0;
}

编译结果：

diego@ubuntu:~/myProg/geeks4geeks/cpp$ gcc test5.c
diego@ubuntu:~/myProg/geeks4geeks/cpp$ g++ test5.c
test5.c: In function 'int main()':
test5.c:5:17: error: invalid conversion from 'void*' to 'int*' [-fpermissive]
int *iptr = vptr; //In C++, it must be replaced with int *iptr=(int *)vptr;

这也就意味着在C++中我们调用malloc时需要显示的转换类型，“int *p = (void *)malloc(sizeof(int))，而在C中不需要。

4) C++中const变量声明的同时必须赋初值，但C中不需要赋初值。

#include <stdio.h>
int main()
{
const int a;   // LINE 4
return 0;
}

编译结果：

diego@ubuntu:~/myProg/geeks4geeks/cpp$ gcc test6.c
diego@ubuntu:~/myProg/geeks4geeks/cpp$ g++ test6.c
test6.c: In function 'int main()':
test6.c:4:15: error: uninitialized const 'a' [-fpermissive]
const int a;   // LINE 4
^

5) C++中的关键字在C中不可用，如new，delete，explicit等。

#include <stdio.h>
int main(void)
{
int new = 5;  // new is a keyword in C++, but not in C
printf("%d", new);
}

6) C++有着更加严格的类型检查。C++无法通过编译，有error，C中仅仅报warning

#include <stdio.h>
int main()
{
char *c = 333;
printf("c = %u", c);
return 0;
}

编译结果：

diego@ubuntu:~/myProg/geeks4geeks/cpp$ gcc test7.c
test7.c: In function 'main':
test7.c:4:15: warning: initialization makes pointer from integer without a cast [enabled by default]
char *c = 333;
^
test7.c:5:5: warning: format '%u' expects argument of type 'unsigned int', but argument 2 has type 'char *' [-Wformat=]
printf("c = %u", c);
^
diego@ubuntu:~/myProg/geeks4geeks/cpp$ g++ test7.c
test7.c: In function 'int main()':
test7.c:4:15: error: invalid conversion from 'int' to 'char*' [-fpermissive]
char *c = 333;
^
test7.c:5:23: warning: format '%u' expects argument of type 'unsigned int', but argument 2 has type 'char*' [-Wformat=]
printf("c = %u", c);
^

[b]7) C++变量可以在任意位置定义，但是C中必须是一个语句块开始的地方。（gcc 中可以编译。。）[/b]

#include <stdio.h>
int main()
{
int i;
i=5;
i++;
int j;
return 0;
}

[b]8) C++存在loop variable，C中不存在。[/b]

#include <stdio.h>
int main()
{
for(int i=0; i<5; i++)
;

return 0;
}

[b]9) C++ main函数必须返回int，C可以返回void[/b]

#include <stdio.h>
void main (int argc, char *argv[])
{
printf("bye\n");
}

编译结果：

diego@ubuntu:~/myProg/geeks4geeks/cpp$ gcc test9.c
diego@ubuntu:~/myProg/geeks4geeks/cpp$ g++ test9.c
test9.c:2:34: error: '::main' must return 'int'
void main (int argc, char *argv[])
^
diego@ubuntu:~/myProg/geeks4geeks/cpp$