Question

I’m trying to initialize a shared_ptr with a map that has optional values. I’ll initialize the values in a later stage of my program.

I read the following post and used it as a guide: How to add valid key without specifying value to a std::map?

But my situation is a little bit different because I’m using a shared_ptr. Without further ado, this is the code I wrote:

ShaderProgram.h

...
#include <map>
#include <boost/shared_ptr.hpp>
#include <boost/optional.hpp>

typedef map<string, optional<GLuint> > attributes_map;

class ShaderProgram
{
public:
    ShaderProgram(vector<string> attributeList);
    ...
private:
    shared_ptr<attributes_map> attributes;
};

ShaderProgram.mm

ShaderProgram::ShaderProgram(vector<string> attributeList)
{
    // Prepare a map for the attributes
    for (vector<string>::size_type i = 0; i < attributeList.size(); i++)
    {
        string attribute = attributeList[i];
        attributes[attribute];
    }
}

The compiler notifies me about the following error: Type ‘shared_ptr’ does not provide a subscript operator.

Anyone an idea what might be the problem?

Answer 1

attributes is a shared_ptr and does not have operator[] but a map does. You need to dereference it:

(*attributes)[attribute];

Note no map object has been allocated for attributes in the constructor so once the compiler error is resolved you will get a runtime failure of some description. Either allocate a map instance:

ShaderProgram::ShaderProgram(vector<string> attributeList) :
    attributes(std::make_shared<attributes_map>())
{
    ...
}

or don’t use a shared_ptr, as it is not obvious why dynamic allocation is required in this case:

private:
    attributes_map attributes;

Pass the attributeList by a reference to avoid unnecessary copying and as const as the constructor does not modify it:

ShaderProgram::ShaderProgram(const vector<string>& attributeList)