Question

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Using array as map value: Cant see the error

Assume I have the following data structure:

std::map<size_t, double[2] > trace;

how can I access its elements with the operator[]?

Essentially I want to do something like:

trace[0][0] = 10.0;
trace[0][1] = 11.0;

In compiling these lines of code I get the following error:

/usr/include/c++/4.6/bits/stl_map.h:453:11: error: conversion from ‘int’ to non-scalar type ‘std::map<long unsigned int, double [2]>::mapped_type {aka double [2]}’ requested

comments?

Answer 1

Value types for maps must be default-constructible, using an expression value_type(), in order to access them via []. For some mysterious reason, array types are not, as specified in C++11 5.3.2/2 (with my emphasis):

The expression T(), where T is a simple-type-specifier or typename-specifier for a non-array complete object type … creates a prvalue of the specified type,which is value-initialized

The compiler gives that strange error when trying to value-initialise an array; the following gives the same error:

typedef double array[2];
array();

I suggest using a class type rather than double[2]: std::array<double,2>, std::pair<double,double>, or a struct containing two doubles. Such types are copyable, don’t decay into a pointer (losing compile-time knowledge of the size) at the drop of a hat, and are generally easier to deal than a built-in array type.

If you’re stuck with an old library that doesn’t provide std::array, then you could use the very similar Boost.Array, or write your own simple class template to wrap an array.