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    <p>There are several cases to consider.</p> <h2>Parameter modified ("out" and "in/out" parameters)</h2> <pre><code>void modifies(T &amp;param); // vs void modifies(T *param); </code></pre> <p>This case is mostly about style: do you want the code to look like <em>call(obj)</em> or <em>call(&amp;obj)</em>? However, there are two points where the difference matters: the optional case, below, and you want to use a reference when overloading operators.</p> <h3>...and optional</h3> <pre><code>void modifies(T *param=0); // default value optional, too // vs void modifies(); void modifies(T &amp;param); </code></pre> <h2>Parameter not modified</h2> <pre><code>void uses(T const &amp;param); // vs void uses(T param); </code></pre> <p>This is the interesting case. The rule of thumb is "cheap to copy" types are passed by value — these are generally small types (but not always) — while others are passed by const ref. However, if you need to make a copy within your function regardless, you <a href="http://cpp-next.com/archive/2009/08/want-speed-pass-by-value/" rel="noreferrer">should pass by value</a>. (Yes, this exposes a bit of implementation detail. <em>C'est le C++.</em>)</p> <h3>...and optional</h3> <pre><code>void uses(T const *param=0); // default value optional, too // vs void uses(); void uses(T const &amp;param); // or optional(T param) </code></pre> <p>There's the least difference here between all situations, so choose whichever makes your life easiest.</p> <h2>Const by value is an implementation detail</h2> <pre><code>void f(T); void f(T const); </code></pre> <p>These declarations are actually the <em>exact same function!</em> When passing by value, const is purely an implementation detail. <a href="http://codepad.org/ReB7ePO8" rel="noreferrer">Try it out:</a></p> <pre><code>void f(int); void f(int const) { /* implements above function, not an overload */ } typedef void NC(int); // typedefing function types typedef void C(int const); NC *nc = &amp;f; // nc is a function pointer C *c = nc; // C and NC are identical types </code></pre>
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    1. POHow to pass objects to functions in C++?
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    1. CO+1 I didn't know about `const` being an implementation when passing by value.
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