C++ class template constructor -- overload reference(U&) with array(U*) failed - c++

Question

I am trying to build a constructor to take an array as an argument which overloads another who take a scalar instead. Code is below.
#include <iostream>
template <typename T>
class SmallVec { // This is a 3 dimensional vector class template
public:
T data[3] = {0}; // internal data of class
template <typename U>
explicit SmallVec(const U& scalar) { // if a scalar, copy it to each element in data
for(auto &item : data) {
item = static_cast<T>(scalar);
}
}
template <typename U>
explicit SmallVec(const U* vec) { // if a vector, copy one by one
for(auto &item : data) {
item = static_cast<T>(*vec);
vec++;
}
}
};
int main() {
float num = 1.2;
float *arr = new float[3];
arr[2] = 3.4;
SmallVec<float> vec1(num); // take num, which works fine
SmallVec<float> vec2(arr); // !!!--- error happens this line ---!!!
std::cout << vec1.data[2] << " " << vec2.data[2] << std::endl;
return 0;
}
The compiler complains that
error: invalid static_cast from type 'float* const' to type 'float'
Obviously, vec2(arr) still calls the first constructor. However, if I remove template <typename U> and replace U to T. The program just works fine. What should I do to correct this?
Any suggestions are appreciated!

Answer 1

Here's how to use SFINAE to get what you want:
#include <vector>
#include <map>
#include <string>
using namespace std;
template<class T>
struct Foo {
template <class U, typename enable_if<is_pointer<U>::value, int>::type = 0>
Foo(U u){}
template <class U, typename enable_if<!is_pointer<U>::value, int>::type = 0>
Foo(U u){}
};
int main()
{
Foo<int> f('a'); // calls second constructor
Foo<int> f2("a"); // calls first constructor
}
live: https://godbolt.org/g/ZPcb5T

Answer 2

I am trying to build a constructor to take an array as an argument
(...)
explicit SmallVec(const U* vec) { // if a vector, copy one by one
You do not take an array. You take a pointer, which may or may not point to an array, and even if it points to an array, who says that the array has at least three elements? That's a serious design flaw.
C++ does allow you to take raw arrays by reference or const reference, even though the syntax is horrible:
explicit SmallVec(const U (&vec)[3]) {
The implementation of the constructor is then also different:
for(int index = 0; index < 3; ++index) {
data[index] = static_cast<T>(vec[index]);
}
Looking at main, however, the problem goes deeper. You use new[] to allocate an array dynamically. That's already a very bad idea. Coincidentally, your example also misses a delete[]. Why don't you use a local array instead?
float arr[3];
This will make your program compile and probably run correctly, but there's still undefined behaviour in your code, because you only set the 3rd element of the array to a valid value; the other two elements remain uninitialised, and reading from an uninitialised float, even if you just copy it, formally results in undefined behaviour.
So better make it:
float arr[3] = { 0.0, 0.0, 3.4 };
In addition to that, C++11 invites you to use std::array, which generally makes things a bit safer and improves the syntax. Here is a complete example:
#include <iostream>
#include <array>
template <typename T>
class SmallVec { // This is a 3 dimensional vector class template
public:
std::array<T, 3> data; // internal data of class
template <typename U>
explicit SmallVec(const U& scalar) { // if a scalar, copy it to each element in data
for(auto &item : data) {
item = static_cast<T>(scalar);
}
}
template <typename U>
explicit SmallVec(std::array<U, 3> const& vec) { // if a vector, copy one by one
for(int index = 0; index < 3; ++index) {
data[index] = static_cast<T>(vec[index]);
}
}
};
int main() {
float num = 1.2;
std::array<float, 3> arr = { 0.0, 0.0, 3.4 };
SmallVec<float> vec1(num);
SmallVec<float> vec2(arr);
std::cout << vec1.data[2] << " " << vec2.data[2] << std::endl;
return 0;
}

Answer 3

Even though both constructors use the explicit specifier and try to avoid type conversions you should note that the first is just as good a candidate as the second. If you substitute U for float* you will get:
explicit SmallVec(const float*& scalar)
which is totally acceptable and will explain the compilation error.
You could resolve the problem by changing the second constructor to:
template <typename U>
explicit SmallVec(U* const vec) { // if a vector, copy one by one
U* local = vec;
for(auto &item : data) {
item = static_cast<T>(*local);
local++;
}
}
However, I suggest an even more explicit way:
class ScalarCopy {};
class VectorCopy {};
...
template <typename U>
SmallVec(const U& vec, ScalarCopy);
template <typename U>
SmallVec(const U* const vec, VectorCopy);
and make explicit calls:
SmallVec<float> vec1(num, ScalarCopy());
SmallVec<float> vec2(arr, VectorCopy());