StackOverflow2013

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POEasy rule to read complicated const declarations?
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copied!<p>For reading complex pointer declarations there is the <a href="http://ieng9.ucsd.edu/~cs30x/rt_lt.rule.html" rel="noreferrer">right-left rule</a>.</p> <p>But this rule does not mention how to read <code>const</code> modifiers.</p> <p>For example in a simple pointer declaration, <code>const</code> can be applied in several ways:</p> <pre><code>char *buffer; // non-const pointer to non-const memory const char *buffer; // non-const pointer to const memory char const *buffer; // equivalent to previous declartion char * const buffer = {0}; // const pointer to non-const memory char * buffer const = {0}; // error const char * const buffer = {0}; // const pointer to const memory </code></pre> <p>Now what about the use of <code>const</code> with a pointer of pointer declaration?</p> <pre><code>char **x; // no const; const char **x; char * const *x; char * * const x; const char * const * x; const char * * const x; const char * const * const x; </code></pre> <p>And what is an easy rule to read those declarations? Which declarations make sense? </p> <p>Is the <a href="http://c-faq.com/decl/spiral.anderson.html" rel="noreferrer">Clockwise/Spiral Rule</a> applicable?</p> <h2>Two real world examples</h2> <p>The method <a href="http://clang.llvm.org/doxygen/classclang_1_1ASTUnit.html#ab18e63a754071c368a0afccbec0de8a8" rel="noreferrer"><code>ASTUnit::LoadFromCommandLine</code></a> uses <code>const char **</code> to supply command line arguments (in the llvm clang source).</p> <p>The argument vector parameter of <a href="http://linux.die.net/man/3/getopt" rel="noreferrer"><code>getopt()</code></a> is declared like this:</p> <pre><code>int getopt(int argc, char * const argv[], const char *optstring); </code></pre> <p>Where <code>char * const argv[]</code> is equivalent to <code>char * const * argv</code> in that context.</p> <p>Since both functions use the same concept (a vector of pointers to strings to supply the arguments) and the declarations differ - the obvious questions are: Why do they differ? Makes one more sense than the other?</p> <p>The intend should be: The <code>const</code> modifier should specify that the function does not manipulate strings of this vector and does not change the structure of the vector.</p>

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