setprecision不显示数字

Question

我有以下功能片段：

bool A_AbstractTester_Actor::Equals( int lineNumber, T_String valueText, double value, T_String expectedText, double expected, double accuracy )
{
    ...
    v_FailedText << std::setprecision(8) << "\nLine " << lineNumber  
     << ": EQUALS_DOUBLE FAILED, Value '" << valueText << "' (" << value 
     << "), Expected '" << expectedText << "' (" << expected << ")\n";

稍后我打印v_FailedText，结果是：

Line 242: EQUALS_DOUBLE_FAILED, Value 'step' (0), Expected '1.0' (1)

我的期望是：

Line 242: EQUALS_DOUBLE_FAILED, Value 'step' (0.00000000), Expected '1.0' (1.00000000)