Getting error: implicit conversion loses integer precision:

#include <string>
#include <vector>
using namespace std;

#include "symboltable.h"

void SymbolTable::insert(string variable, int value)
{
    const Symbol& symbol = Symbol(variable, value);
    elements.push_back(symbol);
}

int SymbolTable::lookUp(string variable) const
{
    for (size_t i = 0; i < elements.size(); i++)
        if (elements[i].variable == variable)
             return elements[i].value;
    return -1;
}

void SymbolTable::init()
{
	if(elements.size() > 0)
	{
		for (int i = elements.size(); i > 0; i--) {
			elements.pop_back();
		}
	}
}

for (int i = elements.size(); i > 0; i--) { *** Error- Implicit conversion loses integer precision: 'std::vector<SymbolTable::Symbol>::size_type' (aka 'unsigned long') to 'int'