const char * const * expressions；这个定义怎么理解？

Question

来源于hyperscan的hs_compile函数的第一个参数

Answer 1

下面代码中

Bjarne在他的The C++ Programming Language里面给出过一个助记的方法：把一个声明从右向左读
char * const cp; ( * 读成 pointer to )
cp is a const pointer to char
const char * p;
p is a pointer to const char;

expressions1 is a pointer to pointer to char
expressions2 is a pointer to pointer to const char
expressions is a pointer to const pointer to const char
expressions 本身可以改动，它指向的指针是不可以改动的，它指向的const 指针指向的 const char也是不可以改动的

 char a[4] = { 'a','a','a','\0' };
    char b[4] = { 'b','a','a','\0' };
    char * p = a;
    char * l = b;
    char * ptr2[2] = {p,l};

    char ** expressions1 = ptr2;
    expressions1[0] = b;
    expressions1[0][0] = 'c';

    const char ** expressions2 = ptr2;
    expressions2[0] = b;
    expressions2[0][0] = 'c';   //编译器报错

    const char * const * expressions = ptr2;
    expressions[0] = b;               //编译器报错
    expressions[0][0] = 'c';          //编译器报错
    expressions = expressions2;