Indexing an std::vector with a negative index

Question

I tried to index a vector using a negative index. The vector::at() member function checks whether the specified index is within the bounds of the vector, and if this does not occur, an out_of_range exception is thrown.

vector<float> array;   // sample vector
array.push_back(0.123);
array.push_back(1.234);
array.push_back(2.345);
array.push_back(3.456);
array.push_back(4.567);

int index = -1;
float f = array.at(index);
cout << f << endl;

The signature of vector::at() member function requires that the specified parameter is of vector<T>::size_type type, and this type is unsigned int for the vector, so the compiler should perform an implicit conversion from int (the type of the index variable) to unsigned int . Since the index value is -1 in the above example, the implicitly converted index is 4294967295 (that is the max value of the unsigned int type): this value is passed to vector::at() member function, which throws an out_of_range exception.

In other words, this exception is not thrown because the vector::at() member function sees that the index is less than zero, but rather because the implicitly converted index is greater than the current size of the vector . Is this a correct explanation?

Answer 1

Yes, that is a correct explation. (Except at takes a vector::size_type , usually std::size_t , which is some unspecified unsigned integer type (usually the bit-width of pointers on your system). It being unsigned int on your system is allowed, but not mandated by the standard; 32 bit unsigned int with 64 bit size_t is common.)

As an aside, be careful with unsigned to signed conversion: the standard does not require it be a round-trip going signed->unsigned->signed for negative values, and some compilers aggressively optimize in unexpected ways.