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Swift 3 

let stackView = UIStackView()
stackView.axis = .horizontal
stackView.alignment = .fill // .leading .firstBaseline .center .trailing .lastBaseline
stackView.distribution = .fill // .fillEqually .fillProportionally .equalSpacing .equalCentering

let label = UILabel()
label.text = "Text"
stackView.addArrangedSubview(label)
// for horizontal stack view, you might want to add width constraint to label or whatever view you're adding.

Swift

let stackView = UIStackView()
stackView.axis = .Horizontal
stackView.alignment = .Fill // .Leading .FirstBaseline .Center .Trailing .LastBaseline
stackView.distribution = .Fill // .FillEqually .FillProportionally .EqualSpacing .EqualCentering

let label = UILabel(frame: CGRectZero)
label.text = "Label"
stackView.addArrangedSubview(label)
// for horizontal stack view, you might want to add width constraint to label or whatever view you're adding.

Objective-C

UIStackView *stackView = [[UIStackView alloc] init];
stackView.axis = UILayoutConstraintAxisHorizontal;
stackView.alignment = UIStackViewAlignmentFill; //UIStackViewAlignmentLeading, UIStackViewAlignmentFirstBaseline, UIStackViewAlignmentCenter, UIStackViewAlignmentTrailing, UIStackViewAlignmentLastBaseline
stackView.distribution = UIStackViewDistributionFill; //UIStackViewDistributionFillEqually, UIStackViewDistributionFillProportionally, UIStackViewDistributionEqualSpacing, UIStackViewDistributionEqualCentering

UILabel *label = [[UILabel alloc] initWithFrame:CGRectZero];
label.text = @"Label";
[stackView addArrangedSubview:label];
//For horizontal stack view, you might want to add a width constraint to your label or whatever view you are adding.