Indexing an std::vector with a negative index

问题:

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?

回答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.