In People v. Gallagher (1893) 100 Cal. 466, Penal Code section 1323 then provided: "A defendant in a criminal action or proceeding cannot be compelled to be a witness against himself; but if he offer himself as a witness he may be cross-examined by counsel for the people as to all matters about which he was examined in chief." ( Id. at p. 475.) Concerning the defendant who testified, the court there expressly held: "By offering himself as a witness he waived all objection to his constitutional right to claim exception from giving testimony against himself upon all the matters about which he should volunteer to testify, and as to those matters he opened the door for the most searching investigation by cross-examination as to the accuracy of his testimony as fully as any other witness who might have given the same testimony." ( Id. at p. 476.) The court further explained that defendant's credibility could be tested even by cross-examination about a fact in issue, which tends to contradict a denial which he has made even though no question was asked of him on direct concerning such fact. ( People v. Gallagher, supra , at pp. 475-476 35 P. 80, at p. 83.)