使用排序时类型无效的初始化

Question

I have the following code to sort a vector of a custom type. It used to work, but after building the code on another system it gives an error at compile time.

The context the sort() call is made.

std::vector<std::vector<AssemblyObject>>*     LegoAssembler::getLayers(std::vector<AssemblyObject> completeAssembly)
{    
    std::vector<std::vector<AssemblyObject>>* layers = new std::vector<std::vector<AssemblyObject>>();
    std::vector<AssemblyObject> cLayer;

    double lastZ = 0;
    std::sort(completeAssembly.begin(), completeAssembly.end(), AssemblyObject::compare);

    ...
}

The sorting function

bool AssemblyObject::compare(AssemblyObject &a, AssemblyObject &b){
return (a.getPosition()[2] < b.getPosition()[2]) ||
       ((a.getPosition()[2] == b.getPosition()[2]) && (a.getPosition()[1] > b.getPosition()[1])) ||
       ((a.getPosition()[2] == b.getPosition()[2]) && (a.getPosition()[1] == b.getPosition()[1]) && (a.getPosition()[0] > b.getPosition()[0]));
}

The Error

/usr/include/c++/4.8/bits/stl_algo.h:2263: error: invalid initialization of reference of type ‘AssemblyObject&’ from expression of type ‘const AssemblyObject’
while (__comp(*__first, __pivot))

/usr/include/c++/4.8/bits/stl_algo.h:2263: error: invalid initialization of reference of type ‘AssemblyObject&’ from expression of type ‘const AssemblyObject’
while (__comp(*__first, __pivot))
                               ^
                               ^

As I said, this happened after building the code on another system. I was thinking it had something to do with changing compiler versions, but then again, I think something as simple as a sort function wouldn't break. Plus I'd like the code to compile on both compilers if that was the case.

Really would appreciate some help,

Answer 1

Your code is attempting to take a non-const reference to a const object, which is not permitted. The compare function doesnt modify its arguments so change:

bool AssemblyObject::compare(AssemblyObject &a, AssemblyObject &b){

To

bool AssemblyObject::compare(const AssemblyObject &a, const AssemblyObject &b){

Answer 2

The error is pretty clear - you need compare to accept const lvalue-references , not mutable ones:

bool AssemblyObject::compare(const AssemblyObject &a, const AssemblyObject &b)
{ 
    /* as before */
}