Guns and knives are not treated exactly the same in France. They are in different categories of weapons. The only real definition of absolutely illegal types of knife is used by French customs, and there you find one-handed opening/automatically locking blades, gravity knives, stilettos, and fixed blades that must be more than 15 cm long, have a substantial hand guard and a double cutting edge to be considered illegal (basically, bayonets). But a knife in general can be considered a 'weapon by intent' and as such be confiscated.
In practice it means that police officers have to make the call when they find you are carrying a knife. If it's in an urban area and they are stressed (say there are riots in town), you will lose your knife, no matter the type or size. In rural areas, the gendarmes are a lot more tolerant, and need to be if they don't want to arrest almost everybody, between farmers, hunters and all the others out looking for mushrooms and whatnot. I have walked into the gendarmerie several times on other matters, while openly carrying my Laguiole and my Leatherman on my belt, and they did not even blink.
Generally, the advice to keep it to a relatively small slipjoint is a good one, especially in the cities.
Guns, by contrast, are just prohibited (unless you are a licensed hunter engaged in a hunt in the countryside). Civilians carry no handguns in France, period. There is no room for interpretation by police officers there, they will just arrest you.
