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copied!<p>No, you can't change it. What do you hope to learn from seeing an implementation of it?</p> <p>What <code>sizeof</code> does can't be written in C++ using more basic operations. It's not a function, or part of a library header like e.g. <code>printf</code> or <code>malloc</code>. It's inside the compiler.</p> <p>Edit: If the compiler is itself written in C or C++, then you can think of the implementation being something like this:</p> <pre><code>size_t calculate_sizeof(expression_or_type) { if (is_type(expression_or_type)) { if (is_array_type(expression_or_type)) { return array_size(exprssoin_or_type) * calculate_sizeof(underlying_type_of_array(expression_or_type)); } else { switch (expression_or_type) { case int_type: case unsigned_int_type: return 4; //for example case char_type: case unsigned_char_type: case signed_char_type: return 1; case pointer_type: return 4; //for example //etc., for all the built-in types case class_or_struct_type: { int base_size = compiler_overhead(expression_or_type); for (/*loop over each class member*/) { base_size += calculate_sizeof(class_member) + padding(class_member); } return round_up_to_multiple(base_size, alignment_of_type(expression_or_type)); } case union_type: { int max_size = 0; for (/*loop over each class member*/) { max_size = max(max_size, calculate_sizeof(class_member)); } return round_up_to_multiple(max_size, alignment_of_type(expression_or_type)); } } } } else { return calculate_sizeof(type_of(expression_or_type)); } } </code></pre> <p>Note that is is very much pseudo-code. There's lots of things I haven't included, but this is the general idea. The compiler probably doesn't actually do this. It probably calculates the size of a type (including a class) and stores it, instead of recalculating every time you write <code>sizeof(X)</code>. It is also allowed to e.g. have pointers being different sizes depending on what they point to.</p>

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