mapnik::projection const *_merc =  new mapnik::projection("+init=epsg:3857");
mapnik::projection const *_wgs84 = new mapnik::projection("+init=epsg:4326");
mapnik::proj_transform const *transformer_ = new mapnik::proj_transform(*_merc, *_wgs84);

std::clog << "before reprojection:\n";
mapnik::coord2d c = bb.center();
std::clog << std::setw(10) << c.y << "," << c.x << "\n";

{
    std::clog << "after reprojection:\n";
    transformer_->backward(bb);
    transformer_->forward(bb);
    mapnik::coord2d c = bb.center();
    std::clog << std::setw(10) << c.y << "," << c.x << "\n";
}

/*
produces output:

before reprojection:
   85.0483,-179.967
after reprojection:
   85.0442,-179.967
*/