Float Values as an index in an Array in C++

Can a float value be开发者_如何学C used as the index of an array? What will happen if an expression used as an index resulted to a float value?

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The float value will be casted to int (it can give warning or error depending on compiler's warning level)

s1 = q[12.2]; // same as q[12]
s2 = q[12.999999]; // same as q[12]
s3 = q[12.1/6.2]; // same as q[1]

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Yes. But it's pointless. The float value will be truncated to an integer.

(You can use std::map<float, T>, however, but most of the time you'll miss the intended values because of inaccuracy.)

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A C++ array is a contiguous sequence of memory locations. a[x] means "the xth memory location after one pointed to by a."

What would it mean to access the 12.4th object in a sequence?

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It will be casted to int.

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This is an error. In [expr.sub]:

A postfix expression followed by an expression in square brackets is a postfix expression. One of the expressions shall have the type “pointer to T” and the other shall have unscoped enumeration or integral type.

I am not aware of a clause in the standard that specifies that conversion should happen here (admittedly, I would not be surprised if such a clause existed), although testing with ideone.com did produce a compilation error.

However, if you're subscripting a class rather than a pointer — e.g. std::vector or std::array — then the overload of operator[] will have the usual semantics of a function call, and floating-point arguments will get converted to the corresponding size_type.